[RISCV] Adjust RISCVVectorMaskDAGMutation to look for copy to V0 (#129296)

This mutation was introduced in 01a15dca0 with the goal of avoiding many
copies from V1-v31 to v0 immediately before a mask consuming
instruction. I noticed in a workload that this was not applying to
vmv.s.x (which we use for short vector masks). We'd had a whitelist of
instructions. Instead, we can directly inspect the user of the current
node to see if it's a copy to V0. This isn't quite precise (as the mask
producing instruction could already be scheduled fairly far from it's
single use), but is probably good enough.

As with all schedule changes, results are mixed. Some significant
improvements, some regressions.

0 files changed, 0 insertions, 0 deletions