AMDGPU: Use default operand for VOP3P clamp

AMDGPU: Use default operand for VOP3P clamp

We don't use this, and matching from the def doesn't make much sense.

There are multiple tablegen bugs with default operand
handling. undef_tied_input should work to handle the vdst_in
correctly, but this breaks the operand register class constraint which
it should be able to infer.