aarch64: Simplify SHRN, RSHRN expanders and patterns

Now that we've got the <vczle><vczbe> annotations we can get rid of explicit
!BYTES_BIG_ENDIAN and BYTES_BIG_ENDIAN patterns for the narrowing shift instructions.
This allows us to clean up the expanders as well.

Bootstrapped and tested on aarch64-none-linux-gnu and aarch64_be-none-elf.

gcc/ChangeLog:

	* config/aarch64/aarch64-simd.md (aarch64_shrn<mode>_insn_le): Delete.
	(aarch64_shrn<mode>_insn_be): Delete.
	(*aarch64_<srn_op>shrn<mode>_vect):  Rename to...
	(*aarch64_<srn_op>shrn<mode><vczle><vczbe>): ... This.
	(aarch64_shrn<mode>): Remove reference to the above deleted patterns.
	(aarch64_rshrn<mode>_insn_le): Delete.
	(aarch64_rshrn<mode>_insn_be): Delete.
	(aarch64_rshrn<mode><vczle><vczbe>_insn): New define_insn.
	(aarch64_rshrn<mode>): Remove references to the above deleted patterns.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/simd/pr99195_5.c: Add testing for shrn_n, rshrn_n
	intrinsics.