[RISCV] Select unmasked RVV pseudos in a DAG post-process

This patch drops TableGen patterns matching all-ones masked RVV pseudos
in the case where there are fallback patterns matching the generic
masked forms to "_MASK" pseudos. This optimization is now performed with
a SelectionDAG post-processing step which peephole-optimizes these same
pseudos with all-ones masks and swaps them out to their unmasked
pseudos.

This cuts our generated ISel table down by around ~5% (~110kB) in lieu
of a far smaller auto-generated table to help with the peephole.

This only targets our custom RISCVISD::*_VL binary operator nodes, which
use the one form for both masked and unmasked variants. A similar
approach could be used for our intrinsics but we'd need to do some work,
e.g., to represent unmasked intrinsics as true-masked intrinsics at the
IR or ISel level. At a rough estimate, this could save us a further 9%
on the size of our ISel table for the binary intrinsic patterns alone.

There is no observable impact on our tests.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D118810