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

[Philip Reames via llvm-commits]

https://github.com/preames commented:

Did a sweep through the test diffs - mostly ignored vp tests, and don't claim to have looked at everything.  We *could* do another round of diff reduction here, but I am not currently asking for it.  I'm going to apply this downstream and look at a couple workloads, another comment will follow once I've done so.

I will note that my comments flag at least one real regression caused/triggered by this patch.  I'm not yet sure that regression *matters* but it does appear to be *real*.  

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