[RISCV] Correct policy operand for RVV ISel patterns with merge operand as implicit_def (NFC)

The trailing TU_MU suffixes was added in D154625. The trailing policy
operand for these patterns has no real affect, as the vsetvli insertion
pass omits the trailing policy operand when the merge operand is
undefined.

This patch is essentially an NFC. However, the policy implied for these
patterns is actually TA_MA. This patch corrects them to avoid confusion.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D156342