[llvm] [PeepholeOptimizer] Add REG_SEQUENCE subregister use folding (PR #205795)

[Frederik Harwath via llvm-commits]

frederik-h wrote:

@arsenm This PR is similar to #188451, but it implements the optimization in peephole-opt instead of si-fold-operands as you suggested. As described in the [comment](https://github.com/llvm/llvm-project/pull/205795#discussion_r3480768514) above, I have some issues with the "undef deprecator". This is the only reason why the PR is still a draft.

https://github.com/llvm/llvm-project/pull/205795