[AArch64] Describe the 'BSL' RTL pattern more accurately.

gcc/
	* config/aarch64/aarch64-simd.md
	(aarch64_simd_bsl<mode>_internal): Rewrite RTL to not use UNSPEC_BSL.
	(aarch64_simd_bsl<mode>): Likewise.
	* config/aarch64/iterators.md (unspec): Remove UNSPEC_BSL.



git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@198316 138bc75d-0d04-0410-961f-82ee72b054a4