There are a lot of messes in the special case
predicate handling. Currently broad let blocks
override specific predicates with more general
cases. For instructions with SDWA, the HasSDWA
predicate was overriding the SubtargetPredicate
for the instruction.

This fixes enough to properly disallow new instructions
that support SDWA on older targets.

Mostly a stub, but adds some baseline tests and
tests for removed instructions.