[Inliner] Remove alloca merging

See https://discourse.llvm.org/t/rfc-we-should-stop-merging-allocas-in-the-inliner/41975 for the RFC from 2016 around this.

Backend stack coloring takes care of what this initially set out to help with.

This only affects the legacy inliner, which for the most part doesn't matter anymore. The new PM inliner never looked at this flag.

There are some backends running the legacy always-inline pass, but that's unlikely to matter.

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D146091