Skip nullcheck frozen object in unbox optimization (#121709)

Fixes the build crash in #121697.

Since with TrimMode=partial, we consider all static fields targets of
reflection, we cannot allow inlining their contents. With full trimming
allowed, the optimization is unlocked and we hit an assert when trying
to nullcheck a constant.

Author: MichalStrehovsky

src/coreclr/jit/importer.cpp

@@ -10490,16 +10490,26 @@ void Compiler::impImportBlockCode(BasicBlock* block)
                                 GenTree* boxPayloadOffset = gtNewIconNode(TARGET_POINTER_SIZE, TYP_I_IMPL);
                                 GenTree* boxPayloadAddress =
                                     gtNewOperNode(GT_ADD, TYP_BYREF, cloneOperand, boxPayloadOffset);
-                                GenTree* nullcheck = gtNewNullCheck(op1);
-                                // Add an ordering dependency between the null
-                                // check and forming the byref; the JIT assumes
-                                // in many places that the only legal null
-                                // byref is literally 0, and since the byref
-                                // leaks out here, we need to ensure it is
-                                // nullchecked.
-                                nullcheck->SetHasOrderingSideEffect();
-                                boxPayloadAddress->SetHasOrderingSideEffect();
-                                GenTree* result = gtNewOperNode(GT_COMMA, TYP_BYREF, nullcheck, boxPayloadAddress);
+
+                                GenTree* result;
+                                if (fgAddrCouldBeNull(op1))
+                                {
+                                    GenTree* nullcheck = gtNewNullCheck(op1);
+                                    // Add an ordering dependency between the null
+                                    // check and forming the byref; the JIT assumes
+                                    // in many places that the only legal null
+                                    // byref is literally 0, and since the byref
+                                    // leaks out here, we need to ensure it is
+                                    // nullchecked.
+                                    nullcheck->SetHasOrderingSideEffect();
+                                    boxPayloadAddress->SetHasOrderingSideEffect();
+                                    result = gtNewOperNode(GT_COMMA, TYP_BYREF, nullcheck, boxPayloadAddress);
+                                }
+                                else
+                                {
+                                    // We don't need a nullcheck if this is e.g. a preinitialized value
+                                    result = boxPayloadAddress;
+                                }
                                 impPushOnStack(result, tiRetVal);
                                 break;
                             }