rs6000/testsuite: Allow xxperm* instead of only vperm*

rs6000/testsuite: Allow xxperm* instead of only vperm*

Some testcases failed (esp. with --with-cpu=power9) after my change to
prefer xxperm over vperm when all else is equal.  Fix that.  (This also
tightens the relevant REs somewhat).

2020-06-09  Segher Boessenkool  <segher@kernel.crashing.org>

gcc/testsuite/
	* gcc.target/powerpc/fold-vec-perm-char.c: Allow both vperm/vpermr and
	xxperm/xxpermr.
	* gcc.target/powerpc/fold-vec-perm-double.c: Ditto.
	* gcc.target/powerpc/fold-vec-perm-float.c: Ditto.
	* gcc.target/powerpc/fold-vec-perm-int.c: Ditto.
	* gcc.target/powerpc/fold-vec-perm-longlong.c: Ditto.
	* gcc.target/powerpc/fold-vec-perm-pixel.c: Ditto.
	* gcc.target/powerpc/fold-vec-perm-short.c: Ditto.
	* gcc.target/powerpc/lvsl-lvsr.c: Ditto.
	* gcc.target/powerpc/vec-mult-char-2.c: Ditto.
	* gcc.target/powerpc/vsx-vector-6.p9.c: Also allow xxpermr.