[RISCV] Remove legacy TA/TU pseudo distinction for VID

This is a follow on to D152740. The focus of this patch is on actually removing the old TA (unsuffixed) version. I realized we already had plumbing for combined TA/TU pseudos - used by some of the ternary instructions. As such, we can go ahead and fully remove the old TA, and rename the _TU variant to be unsuffixed. (The rename must happen in this patch for the table structure to work out as expected.)

The scheduling difference comes from an omission in D152740. If we selected a _MASK variant - either from manual ISEL or instrincs - we were going through doPeepholeMaskedRVV and still getting the TA variant. The use of the IsCombined flag in the MaskedPseudo table causes us to use the TU (now unsuffixed) variant instead.

Differential Revision: https://reviews.llvm.org/D153155