AMDGPU: Check for other defs when folding conditions into s_andn2_b64

We can't fold the masked compare value through the select if the
select condition is re-defed after the and instruction. Fixes a
verifier error and trying to use the outgoing value defined in the
block.

I'm not sure why this pass is bothering to handle physregs. It's
making this more complex and forces extra liveness computation.