aarch64: Reimplement RSHRN2 intrinsic patterns with standard RTL codes

aarch64: Reimplement RSHRN2 intrinsic patterns with standard RTL codes

Similar to the previous patch, we can reimplement the rshrn2 patterns using standard RTL codes
for shift, truncate and plus with the appropriate constants.
This allows us to get rid of UNSPEC_RSHRN entirely.

Bootstrapped and tested on aarch64-none-linux-gnu.

gcc/ChangeLog:

	* config/aarch64/aarch64-simd.md (aarch64_rshrn2<mode>_insn_le):
	Reimplement using standard RTL codes instead of unspec.
	(aarch64_rshrn2<mode>_insn_be): Likewise.
	(aarch64_rshrn2<mode>): Adjust for the above.
	* config/aarch64/aarch64.md (UNSPEC_RSHRN): Delete.