Make isIsolatedAbove robuster to invalid IR

This function is only called from the verifier.

PiperOrigin-RevId: 267145495

Author: jpienaar

lib/IR/Region.cpp

@@ -143,6 +143,15 @@ static bool isIsolatedAbove(Region &region, Region &limit,
     for (Block &block : *pendingRegions.pop_back_val()) {
       for (Operation &op : block) {
         for (Value *operand : op.getOperands()) {
+          // operand should be non-null here if the IR is well-formed. But
+          // we don't assert here as this function is called from the verifier
+          // and so could be called on invalid IR.
+          if (!operand) {
+            if (noteLoc)
+              op.emitOpError("block's operand not defined").attachNote(noteLoc);
+            return false;
+          }
+
           // Check that any value that is used by an operation is defined in the
           // same region as either an operation result or a block argument.
           if (operand->getParentRegion()->isProperAncestor(&limit)) {