[RISCV] Teach performCombineVMergeAndVOps to handle instructions FMA instructions.

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.

Reviewed By: fakepaper56, frasercrmck

Differential Revision: https://reviews.llvm.org/D151596