RISC-V: Support FP16 for RVV VRGATHEREI16 intrinsic

RISC-V: Support FP16 for RVV VRGATHEREI16 intrinsic

This patch would like to add FP16 support for the VRGATHEREI16
intrinsic. Aka:

* __riscv_vrgatherei16_vv_f16mf4
* __riscv_vrgatherei16_vv_f16mf4_m

As well as f16mf2 to f16m8 types.

Signed-off-by: Pan Li <pan2.li@intel.com>
gcc/ChangeLog:

	* config/riscv/riscv-vector-builtins-types.def
	(vfloat16mf4_t): Add FP16 intrinsic def.
	(vfloat16mf2_t): Ditto.
	(vfloat16m1_t): Ditto.
	(vfloat16m2_t): Ditto.
	(vfloat16m4_t): Ditto.
	(vfloat16m8_t): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/intrisinc-vrgatherei16.c: New test.