Shortcut identity VEC_PERM expansion [PR94710]

Shortcut identity VEC_PERM expansion [PR94710]

This PR is about the rs6000 backend emitting wrong assembly
for whole vector shift by 0, and while I think it is desirable
to fix the backend, I don't see a point why the expander should
try to emit that, whole vector shift by 0 is identity, we can just
return the operand.

2020-04-23  Jakub Jelinek  <jakub@redhat.com>

	PR target/94710
	* optabs.c (expand_vec_perm_const): For shift_amt const0_rtx
	just return v2.