This moves the recognition of GREVI and GORCI from TableGen patterns
into a DAGCombine. This is done primarily to match "deeper" patterns in
the future, like (grevi (grevi x, 1) 2) -> (grevi x, 3).
TableGen is not best suited to matching patterns such as these as the compile
time of the DAG matchers quickly gets out of hand due to the expansion of
commutative permutations.
Could we match i32 pre-type legalization on RV64 and type legalize i32 RISCVISD::GREVI to GREVIW? I think that would remove the need to match from SIGN_EXTEND_INREG.