[ConstFold] Add test for incorrect gep inbounds of undef fold (NFC)

If the offset is zero, then returning poison here is not correct.

Author: nikic

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

@@ -104,14 +104,14 @@ 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)
+; CHECK-NEXT:    call void (...) @use(i1 poison, i1 poison, i8 poison, i8 poison, ptr poison, ptr poison, ptr poison)
 ; CHECK-NEXT:    ret void
 ;
   %i1 = icmp eq i8 poison, 1
   %i2 = fcmp oeq float poison, 1.0
   %i3 = select i1 poison, i8 1, i8 2
   %i4 = select i1 true, i8 poison, i8 %x
-  call void (...) @use(i1 %i1, i1 %i2, i8 %i3, i8 %i4, ptr getelementptr (i8, ptr poison, i64 1), ptr getelementptr inbounds (i8, ptr undef, i64 1))
+  call void (...) @use(i1 %i1, i1 %i2, i8 %i3, i8 %i4, ptr getelementptr (i8, ptr poison, i64 1), ptr getelementptr inbounds (i8, ptr undef, i64 0), ptr getelementptr inbounds (i8, ptr undef, i64 1))
   ret void
 }