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

[Philip Reames via llvm-commits]

preames wrote:

Now that https://github.com/llvm/llvm-project/pull/72096 has landed, the next step is a change which enables the split RA by default.  Once that's in, this change can be rebased on tip of tree, and we can focus review attention on the changes to InsertVSETVLI and the resulting test changes.

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