[AArch64] Simplify+improve patterns for ushr(d?)_n_u64 intrinsic

[AArch64] Simplify+improve patterns for ushr(d?)_n_u64 intrinsic

gcc/:
	* config/aarch64/aarch64-simd.md (aarch64_lshr_simddi): Handle shift
	by 64 by moving const0_rtx.
	(aarch64_ushr_simddi): Delete.

	* config/aarch64/aarch64.md (enum unspec): Delete UNSPEC_USHR64.

gcc/testsuite/:

        * gcc.target/aarch64/ushr64_1.c: Remove scan-assembler "ushr...64".

From-SVN: r218868