[InstSimplify] Fold gep inbounds undef to undef instead of poison

With the semantics change from D154051, it is no longer valid to
fold gep inbounds undef to poison (unless we know the index is
non-zero). Fold it to undef instead.

Differential Revision: https://reviews.llvm.org/D154215

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -4896,9 +4896,9 @@ static Value *simplifyGEPInst(Type *SrcTy, Value *Ptr,
       any_of(Indices, [](const auto *V) { return isa<PoisonValue>(V); }))
     return PoisonValue::get(GEPTy);
 
+  // getelementptr undef, idx -> undef
   if (Q.isUndefValue(Ptr))
-    // If inbounds, we can choose an out-of-bounds pointer as a base pointer.
-    return InBounds ? PoisonValue::get(GEPTy) : UndefValue::get(GEPTy);
+    return UndefValue::get(GEPTy);
 
   bool IsScalableVec =
       isa<ScalableVectorType>(SrcTy) || any_of(Indices, [](const Value *V) {

llvm/test/Transforms/InstSimplify/gep.ll

@@ -160,7 +160,7 @@ define ptr @test7(ptr %b, ptr %e) {
 
 define ptr @undef_inbounds_var_idx(i64 %idx) {
 ; CHECK-LABEL: @undef_inbounds_var_idx(
-; CHECK-NEXT:    ret ptr poison
+; CHECK-NEXT:    ret ptr undef
 ;
   %el = getelementptr inbounds i64, ptr undef, i64 %idx
   ret ptr %el
@@ -176,7 +176,7 @@ define ptr @undef_no_inbounds_var_idx(i64 %idx) {
 
 define <8 x ptr> @undef_vec1() {
 ; CHECK-LABEL: @undef_vec1(
-; CHECK-NEXT:    ret <8 x ptr> poison
+; CHECK-NEXT:    ret <8 x ptr> undef
 ;
   %el = getelementptr inbounds i64, ptr undef, <8 x i64> undef
   ret <8 x ptr> %el