TwoAddrInstructionPass::tryInstructionTransform() has a case where it calls
itself recursively with a new instruction that has not been finalized, in order
to determine whether to keep the instruction. On 'make check' and test-suite the
only cases where the recursive invocation made any transformations were simple
instruction commutations, so I am restricting the recursive invocation to do
only this.

The other cases wouldn't work correctly when updating LiveIntervals, since the
new instructions don't have slot indices and LiveIntervals hasn't yet been
updated. If the other transformations were actually triggering in any test case
it would be possible to support it with a lot of effort, but since they don't
it's not worth it.
TwoAddressInstructionPass::tryInstructionTransform() only potentially returns
true when shouldOnlyCommute is false, so we can remove code that checks
otherwise.