Fix incorrect codegen with respect to GEPs #85333 (#92047)

As mentioned in #68882 and
https://discourse.llvm.org/t/rfc-replacing-getelementptr-with-ptradd/68699

Gep arithmetic isn't consistent with different types. GVNSink didn't
realize this and sank all geps as long as their operands can be wired
via PHIs
in a post-dominator.

Fixes: #85333
Reapply: #88440 after fixing the non-determinism issues in #90995