[InstCombine] return the result of the GEP of PHI transformation early

[danilaml]

Summary

Without this change this function could return nullptr if no further
transformation took place making InstCombine pass incorrectly report no
IR changes preventing analyses invalidation.

[danilaml]

Discovered when some downstream analysis wasn't invalidated when this transformation was the only change instcombine made leading to a miscompile. Not sure how to write a good test for this. I remember there being a pass that verified that passes correctly reported changed IR but I think it only worked for legacy pm.

[nikic]

LG, I don't think we need a test for this.

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

if (DI == -1) {
       for (auto &I : PN->operands())
         NewPN->addIncoming(cast<GEPOperator>(I)->getOperand(DI),
                            PN->getIncomingBlock(I));
 
       NewGEP->setOperand(DI, NewPN);
     }
 
     NewGEP->insertAt(GEP.getParent(), GEP.getParent()->getFirstInsertionPt());
-    replaceOperand(GEP, 0, NewGEP);
-    PtrOp = NewGEP;
+    return replaceOperand(GEP, 0, NewGEP);
   }
 
   if (auto *Src = dyn_cast<GEPOperator>(PtrOp))
     if (Instruction *I = visitGEPOfGEP(GEP, Src))
       return I;
 
   // Skip if GEP source element type is scalable. The type alloc size is unknown
   // at compile-time.