[InstSimplify] Update GEP test to use opaque pointers (NFC)

With opaque pointers, we end up merging these GEPs and dropping
the inrange attribute (in the last two cases). This did not happen
previously, because typed pointers use less powerful GEP folding logic.

I'm a bit unsure whether this is something we need to be concerned
about or not. I believe that generally our stance is that we should
perform folds even if this requires losing poison-generating flags
like inrange.

We can either a) accept this as-is, b) try to inhibit folding if it
requires dropping inrange or c) try to fold to poison if we know
that inrange is going to be violated.

For now, we accept it as-is.

Differential Revision: https://reviews.llvm.org/D127503