[RISCV] Support peephole optimization to fold vmerge.vvm that has tail...

[RISCV] Support peephole optimization to fold vmerge.vvm that has tail agnostic policy and unmasked intrinsics.

This patch supports the tail agnostic part of D130442.

Reviewed By: fakepaper56

Differential Revision: https://reviews.llvm.org/D132923