[RISCV] Use a DAG combine to prune pointless vrgather.vi (#135392)

If the vrgather.vi is preceeded by a vmv.v.x which writes a superset of
the lanes writen by the vrgather, and the vrgather has no passthru, then
the vrgather has no semantic effect.

This is the start of a mini-series of patches around rewriting
vrgather.vi/vx preceeded by vmv.v.x, vfmf.v.f, vmv.s.x, etc... Starting
with the simplest, but also lowest impact.

One point I'd like a second oppinion on is the out of bounds semenatic
change. As far as I can tell, all the indices are in bounds by
construction. The doc change is as much as I couldn't figure out how to
test the alternative as anything else.

0 files changed, 0 insertions, 0 deletions