rs6000, Add test 128-bit shifts for just the int128 type.

rs6000, Add test 128-bit shifts for just the int128 type.

This patch also renames and moves the VSX_TI iterator from vsx.md to
VEC_TI in vector.md.  The uses of VEC_TI are also updated.

2021-04-29  Carl Love  <cel@us.ibm.com>

gcc/ChangeLog

	* config/rs6000/altivec.md (altivec_vslq, altivec_vsrq):
	Rename to altivec_vslq_<mode>, altivec_vsrq_<mode>, mode VEC_TI.
	* config/rs6000/vector.md (VEC_TI): Was named VSX_TI in vsx.md.
	(vashlv1ti3): Change to vashl<mode>3, mode VEC_TI.
	(vlshrv1ti3): Change to vlshr<mode>3, mode VEC_TI.
	* config/rs6000/vsx.md (VSX_TI): Remove define_mode_iterator. Update
	uses of VSX_TI to VEC_TI.

gcc/testsuite/ChangeLog

	* gcc.target/powerpc/int_128bit-runnable.c: Add shift_right, shift_left
	tests.