[llvm] [RISCV] Support postRA vsetvl insertion pass (PR #70549)

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

LGTM. I didn't look at all the test diffs, but in the ones I did I didn't spot anything interesting in them.

Some spills are removed, some new ones are added, but after looking at the benchmark results I don't think they're an issue. Same too goes for the fluctuation in copies.

This is a big change though so probably best to wait for a second LGTM too!

https://github.com/llvm/llvm-project/pull/70549