[ConstFold] Fix incorrect gep inbounds of undef fold

gep inbounds of undef can only be folded to poison if we know
that the offset is non-zero. I don't think precise handling here
is important, so just drop the inbounds special case. This matches
what InstSimplify does.

Author: nikic

llvm/lib/IR/ConstantFold.cpp

@@ -1549,8 +1549,7 @@ Constant *llvm::ConstantFoldGetElementPtr(Type *PointeeTy, Constant *C,
     return PoisonValue::get(GEPTy);
 
   if (isa<UndefValue>(C))
-    // 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);
 
   auto IsNoOp = [&]() {
     // Avoid losing inrange information.

llvm/test/Transforms/InstSimplify/ConstProp/poison.ll

@@ -104,7 +104,7 @@ define void @vec_aggr_ops() {
 
 define void @other_ops(i8 %x) {
 ; CHECK-LABEL: @other_ops(
-; CHECK-NEXT:    call void (...) @use(i1 poison, i1 poison, i8 poison, i8 poison, ptr poison, ptr poison, ptr poison)
+; CHECK-NEXT:    call void (...) @use(i1 poison, i1 poison, i8 poison, i8 poison, ptr poison, ptr undef, ptr undef)
 ; CHECK-NEXT:    ret void
 ;
   %i1 = icmp eq i8 poison, 1