Previously we only handled instructions with merge ops that were
also masked. This patch supports instructions with merge ops that
aren't masked, like FMA.
I'm only folding into a TU vmerge for now. Supporting TA vmerge
shouldn't be much more work, but we need to make sure we get the
policy operand for the result correct. And of course we need more
tests.
Was this moved just to keep the "HasOp" checks together? I don't mind the change - just asking in case it was part of an earlier diff that got refactored.