AArch64: Emit half-precision FCMP/FCMPE

AArch64: Emit half-precision FCMP/FCMPE

Enable a target with FEAT_FP16 to emit the half-precision variants
of FCMP/FCMPE.

gcc/ChangeLog:

	* config/aarch64/aarch64.md: Update cbranch, cstore, fcmp
	and fcmpe to use the GPF_F16 iterator for floating-point
	modes.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/_Float16_cmp_1.c: New test.
	* gcc.target/aarch64/_Float16_cmp_2.c: New (negative) test.