[RISCV] Canonicalize towards vmerge w/passthrough representation

[RISCV] Canonicalize towards vmerge w/passthrough representation

This is the first patch in a series to change how we represent tail agnostic, tail undefined, and tail undisturbed operations. In current code, we tend to use an unsuffixed pseudo for undefined (despite calling it TA most places in code), and the _TU form for both agnostic and undisturbed (via the policy operand).

The key observation behind this patch is that we can represent tail undefined via a pseudo with a passthrough operand if that operand is IMPLICIT_DEF (aka undef). We already have a few instances of this in tree - see vmv.s.x and vslide* - but we can do this more universally. Once complete, we will be able to delete roughly ~1/3 of our vector pseudo classes.

A bit more information on the overall goal can be found in this discourse post: https://discourse.llvm.org/t/riscv-transition-in-vector-pseudo-structure-policy-variants/71295.

This patch doesn't actually remove the legacy unsuffixed pseudo as there's still some path from intrinsic lowering which uses it. (I have not yet located it.) This also means we don't have to modify any of the lookup tables which makes the migration simpler. We can defer deleting the tables and pseudos until one final change once all the instructions have been migrated.

There are a couple of regressions in the tests. At first, these concerned me, but it turns out that all of them are differences in expansion of a single source level instruction. I think we can safely ignore this for the moment. I did explore changing the handling of IMPLICIT_DEF in ScheduleDAG, but that causes an absolutely *massive* test diff with minimal profit. I really don't think it's worth doing.

Differential Revision: https://reviews.llvm.org/D152380