AArch64: remove aarch64_simd_vec_unpack<su>_lo_

The fix for PR18127 reworked the uxtl to zip optimization.
In doing so it undid the changes in aarch64_simd_vec_unpack<su>_lo_ and this now
no longer matches aarch64_simd_vec_unpack<su>_hi_.  It still works because the
RTL generated by aarch64_simd_vec_unpack<su>_lo_ overlaps with the general zero
extend RTL and so because that one is listed before the lo pattern recog picks
it instead.

This removes aarch64_simd_vec_unpack<su>_lo_.

gcc/ChangeLog:

	* config/aarch64/aarch64-simd.md
	(aarch64_simd_vec_unpack<su>_lo_<mode>): Remove.
	(vec_unpack<su>_lo_<mode): Simplify.
	* config/aarch64/aarch64.cc (aarch64_gen_shareable_zero): Update
	comment.