[llvm] PeepholeOpt: Allow introducing subregister uses on reg_sequence (PR #127052)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> LLVM Buildbot has detected a new failure on builder `llvm-clang-x86_64-expensive-checks-ubuntu` running on `as-builder-4` while building `llvm` at step 6 "test-build-unified-tree-check-all".
> 
> Full details are available at: https://lab.llvm.org/buildbot/#/builders/187/builds/4471
> 
> Here is the relevant piece of the build log for the reference

Expensive check failure fixed by #128279

https://github.com/llvm/llvm-project/pull/127052