[RISCV] Lower GREVI and GORCI as custom nodes

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.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D91259