[RISCV] Canonicalize towards vid w/passthrough representation

This patch teaches performCombineVMergeAndVOps how to handle a True instruction (the one being merged into) which is a _TU psuedo, but with an implicit_def passthrough operand.  These are semantically equivalent to the unsuffixed "TA" psuedos, and we can hnndle them as such.

This is a companion to D152380, and demonstrates the unsuffixed to _TA pseudo transition for a non-VMERGE case. Between the two of them, these should cover all the changes required to the post-ISEL combines, and other arithmetic-like instructions should be just TD changes.

See https://discourse.llvm.org/t/riscv-transition-in-vector-pseudo-structure-policy-variants/71295 for context on the patch series.

Differential Revision: https://reviews.llvm.org/D152740