s390: Recognize further vpdi and vmr{l,h} pattern

s390: Recognize further vpdi and vmr{l,h} pattern

Deal with cases where vpdi and vmr{l,h} are still applicable if the
operands of those instructions are swapped.  For example, currently for

V2DI foo (V2DI x)
{
  return (V2DI) {x[1], x[0]};
}

the assembler sequence

vlgvg   %r1,%v24,1
vzero   %v0
vlvgg   %v0,%r1,0
vmrhg   %v24,%v0,%v24

is emitted.  With this patch a single vpdi is emitted.

Extensive tests are included in a subsequent patch of this series where
more cases are covered.

gcc/ChangeLog:

	* config/s390/s390.cc (expand_perm_with_merge): Deal with cases
	where vmr{l,h} are still applicable if the operands are swapped.
	(expand_perm_with_vpdi): Likewise for vpdi.