[InstCombine] Restrict a GEP transform to avoid changing provenance

This is an alternative to D98120. Herein, instead of deleting the transformation entirely, we check
that the underlying objects are both the same and therefore this transformation wouldn't incur a
provenance change, if applied.

https://alive2.llvm.org/ce/z/SYF_yv

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D98588