Perl_rpeep: Fix empty else {} optimization in OP_COND_EXPR case

Until `else` blocks were subject to the same block scoping rules as `if`
and `elsif`, an empty `else` block or ternary condition would arrive at
the peephole optimizer as a bare stub OP:

    +--null (ex-stub) OP(0x562e555a7310)
        FLAGS = (...,PARENS,SLABBED)

Now, it could arrive in that form OR as a SCOPE + NULL:

    scope LISTOP(0x5603b282b190)
    PARENT ===> 3 [cond_expr 0x5603b282b770]
    FLAGS = (VOID,KIDS,SLABBED)
    |
    +--null (ex-stub) OP(0x5603b282b350) ===> 1 [scope 0x5603b282b190]
        FLAGS = (VOID,SLABBED)

This commit updates the "empty else" optimization on `OP_COND_EXPR`s so
that the OPs associated with empty "else" conditions are removed (when
not in scalar context) regardless of which form they arrive in.

Author: richardleach

op.c

@@ -4552,6 +4552,13 @@ Perl_op_scope(pTHX_ OP *o)
                 kid = OpSIBLING(kid);
                 if (kid && OP_TYPE_IS_COP_NN(kid))
                     op_null(kid);
+            } else if (OP_TYPE_IS(kid, OP_STUB)) {
+                OP *sib = OpSIBLING(kid);
+                if (OP_TYPE_IS(sib, OP_NULL) && !OpSIBLING(sib) &&
+                    !(sib->op_flags & OPf_KIDS)) {
+                    op_sibling_splice(o,kid,1,NULL);
+                    op_free(sib);
+                }
             }
         }
         else

peep.c

@@ -3645,10 +3645,10 @@ Perl_rpeep(pTHX_ OP *o)
                 /* Is there an empty "else" block or ternary false branch?
                    If so, optimise away the OP_STUB if safe to do so. */
                 stub = o->op_next;
-                if ((stub->op_flags & OPf_WANT) != OPf_WANT_SCALAR) {
+                if ((stub == falseop) && (stub->op_flags & OPf_WANT) != OPf_WANT_SCALAR) {
                     if (stub->op_type == OP_STUB && !OpSIBLING(stub) ){
                         OP *stubsib = OpSIBLING(stub);
-                        if ((stub == falseop) && !stubsib) {
+                        if (!stubsib) {
                             /*     cond_expr
                              *         -condition-
                              *         - if -
@@ -3659,41 +3659,25 @@ Perl_rpeep(pTHX_ OP *o)
                             o->op_next = stub->op_next;
                             op_sibling_splice(o, OpSIBLING(cLOGOP->op_first), 1, NULL);
                             op_free(stub);
+                        }
                     } else { /* Unexpected */ }
-                } else if (OP_TYPE_IS(stub,OP_ENTER) &&
-                               OP_TYPE_IS(falseop, OP_LEAVE)) {
-                        OP *enter = stub;
-                        OP *stub = OpSIBLING(enter);
-                        if (stub && OP_TYPE_IS(stub, OP_STUB) ){
-                            assert(!(stub->op_flags & OPf_KIDS));
-                            OP *stubsib = OpSIBLING(stub);
-                            assert(stubsib);
-                            if (OP_TYPE_IS(stubsib, OP_NULL) &&
-                                !OpSIBLING(stubsib) &&
-                                !(stubsib->op_flags & OPf_KIDS) ) {
-                                    /*     cond_expr
-                                     *         -condition-
-                                     *         - if -
-                                     *         leave
-                                     *             enter
-                                     *             stub
-                                     *             null
-                                     */
-                            /* Ignoring it for now, pending further exploration.*/
-                            /*
-                                o->op_flags |= OPf_SPECIAL; // For B::Deparse
+                } else if (OP_TYPE_IS(falseop,OP_SCOPE) &&
+                           (falseop->op_flags & OPf_KIDS)) {
+                    OP* stub = cLISTOPx(falseop)->op_first;
+                    if (OP_TYPE_IS_OR_WAS(stub, OP_STUB) && !OpSIBLING(stub)) {
+                        assert(o->op_next != falseop);
+                        assert(!(stub->op_flags & OPf_KIDS));
+                        o->op_flags |= OPf_SPECIAL; /* For B::Deparse */
+                        op_sibling_splice(o, OpSIBLING(cLOGOP->op_first), 1, NULL);
+                        if (o->op_next == stub) {
+                            if (stub->op_next == falseop)
                                 o->op_next = falseop->op_next;
-                                op_sibling_splice(o, OpSIBLING(cLOGOP->op_first), 1, NULL);
-                                op_free(enter);
-                                op_free(stub);
-                                op_free(stubsib);
-                                op_free(falseop);
-                             */
-                            }
+                            else
+                                o->op_next = stub->op_next;
                         }
+                        op_free(falseop);
                     }
                 }
-
             }
             /* FALLTHROUGH */
         case OP_MAPWHILE: