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

[nikic]

Summary

With the semantics change from D154051, it is no longer valid to fold gep inbounds undef to poison (unless we know the index is non-zero).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

Depends on D154051.

[nlopes]

LGTM

llvm/lib/Analysis/InstructionSimplify.cpp

if (Ptr->getType()->getScalarType()->isOpaquePointerTy() &&
     return Ptr;
 
   // getelementptr poison, idx -> poison
   // getelementptr baseptr, poison -> poison
   if (isa<PoisonValue>(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) {
         return isa<ScalableVectorType>(V->getType());
       });
 
   if (Indices.size() == 1) {
     // getelementptr P, 0 -> P.

llvm/test/Transforms/InstSimplify/gep.ll

;
   %b_ptr = ptrtoint ptr %b to i64
   %sub = sub i64 %e_ptr, %b_ptr
   %gep = getelementptr inbounds i8, ptr %b, i64 %sub
   ret ptr %gep
 }
 
 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
 }
 
 define ptr @undef_no_inbounds_var_idx(i64 %idx) {
 ; CHECK-LABEL: @undef_no_inbounds_var_idx(
 ; CHECK-NEXT:    ret ptr undef
 ;
   %el = getelementptr i64, ptr undef, i64 %idx
   ret ptr %el
 }
 
 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
 }
 
 define <8 x ptr> @undef_vec2() {
 ; CHECK-LABEL: @undef_vec2(
 ; CHECK-NEXT:    ret <8 x ptr> undef