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

Following D118810 that reduced the size of ISel table,
this patch optimizes allone-masked RVV pseudos with TU policy and
swap them out to their unmasked TU pseudos.

Since the UNDEF merge operand is not preserved, we turn it into TA
pseudo regardless of the policy operand.

Reviewed By: craig.topper, frasercrmck
Differential Revision: https://reviews.llvm.org/D121881