[llvm] [RISCV] Allow hoisting VXRM writes out of loops speculatively (PR #110044)

[via llvm-commits]

JeffreyALaw wrote:

So just to chime in since I suggested Felipe tackle this problem.

As Felipe noted some designs flush their pipelines on a VXRM write which can be a significant performance issue.  I measured a 2-3% improvement with a patch tackling this problem for GCC running spec2017's x264 workloads on the BPI-F3 board.   That all comes from speculative hoisting of VXRM assignments needed to use vaaddu to implement the ceiling average found in pixel_avg.

https://github.com/llvm/llvm-project/pull/110044