[RISCV] Trim RVV isel pats matchable via DAG post-process

In D122512, several masked patterns were added to support lowering of
vector-predicated float-to-int and int-to-float conversions. With the
introduction of these patterns, all of the old "unmasked" patterns are
matchable via the DAG post-process introduced in D118810, once the relevant
opcode entries are set up in the helper table.

Locally this reduces the generated isel table by 4%.

Reviewed By: arcbbb

Differential Revision: https://reviews.llvm.org/D122637