[CodeGen] Fix nomerge attribute not working in tail calls.

In D79537, `nomerge` was made to only apply to non-tail calls. This fixes it by also applying it to tail calls.

For ARM, I only made the new MI to inherit the flag under `TCRETURNdi` and `TCRETURNri`, because that's the place tail calls got replaced. Not sure if there's any other place needed.

Fixes #61545.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D146749