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.

gcc/ChangeLog

 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.
+
 	PR middle-end/94724
 	* tree.c (get_narrower): Instead of creating COMPOUND_EXPRs
 	temporarily with non-final second operand and updating it later,

gcc/optabs.c

@@ -5627,6 +5627,8 @@ expand_vec_perm_const (machine_mode mode, rtx v0, rtx v1,
       if (shift_amt)
 	{
 	  class expand_operand ops[3];
+	  if (shift_amt == const0_rtx)
+	    return v2;
 	  if (shift_code != CODE_FOR_nothing)
 	    {
 	      create_output_operand (&ops[0], target, mode);