[RISCV] Match RVV VF variants also through masked operations

[RISCV] Match RVV VF variants also through masked operations

This brings floating-point RVV vector/scalar support more in line with
the integer vector patterns, which can already match '.vx' instructions
with masked operations.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D117697