aarch64: Use standard RTL codes for __rev16 intrinsic expansion

aarch64: Use standard RTL codes for __rev16 intrinsic expansion

I noticed for the expansion of the __rev16* arm_acle.h intrinsics we don't need to use an unspec just because it doesn't match neatly to a bswap code.
We have organic combine patterns for it that we can reuse.
This patch removes the define_insn using UNSPEC_REV (should it have been an UNSPEC_REV16?) and adds an expander to emit
the patterns we have for rev16 using standard RTL codes.

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

gcc/ChangeLog:

	* config/aarch64/aarch64.md (@aarch64_rev16<mode>): Change to
	define_expand.
	(rev16<mode>2): Rename to...
	(aarch64_rev16<mode>2_alt1): ... This.
	(rev16<mode>2_alt): Rename to...
	(*aarch64_rev16<mode>2_alt2): ... This.