[RISCV] Remove legacy TA/TU pseudo distinction of vmerge and carry-in arithmetic operations [NFC[

his change continues with the line of work discussed in https://discourse.llvm.org/t/riscv-transition-in-vector-pseudo-structure-policy-variants/71295.

This is analogous to other patches in the series, but with one key difference - the resulting pseudo does *not* have a policy operand. We could add one for vmerge, but the some of the multiclasses are sufficiently entwined with the mask producing arithmetic instructions that the change delta becomes unmanageable. Note that these instructions are *not* in the RISCVMaskedPseudo table, and thus the difference doesn't complicate other code. The main value of working incrementally here is that we get to eagerly cleanup the IsTA logic flowing through the post-ISEL combines.

Differential Revision: https://reviews.llvm.org/D154645