Fixes selection for scalar G_SMULH/G_UMULH. Also switches to using
tablegen selected add/sub, which switch to the signed version of the
opcode. This matches the current DAG behavior. We can't drop the
manual selection for add/sub yet, because it's still both for VALU
add/sub and for G_PTR_ADD.
The other 3-op patterns should also be theoretically handled, but
currently there's a bug in the inferred pattern complexity.
I'm not sure what the error handling strategy should be for potential
constant bus violations. I think the correct strategy is to never
produce mixed SGPR and VGPR operands in a typical VOP instruction,
which will trivially avoid them. However, it's possible to still have
hand written MIR (or erroneously transformed code) with these
operands. When these fold, the restriction will be violated. We
currently don't have any verifiers for reg bank legality. For now,
just ignore the restriction.
It might be worth triggering a DAG fallback on verifier error.
Matt Arsenault