arm: Auto-vectorization for MVE: add __fp16 support to VCMP

arm: Auto-vectorization for MVE: add __fp16 support to VCMP

This patch adds __fp16 support to the previous patch that added vcmp
support with MVE. For this we update existing expanders to use VDQWH
iterator, and add a new expander vcond<VH_cvtto><mode>.  In the
process we need to create suitable iterators, and update v_cmp_result
as needed.

2021-05-17  Christophe Lyon  <christophe.lyon@linaro.org>

	gcc/
	* config/arm/iterators.md (V16): New iterator.
	(VH_cvtto): New iterator.
	(v_cmp_result): Added V4HF and V8HF support.
	* config/arm/vec-common.md (vec_cmp<mode><v_cmp_result>): Use VDQWH.
	(vcond<mode><mode>): Likewise.
	(vcond_mask_<mode><v_cmp_result>): Likewise.
	(vcond<VH_cvtto><mode>): New expander.

	gcc/testsuite/
	* gcc.target/arm/simd/mve-compare-3.c: New test with GCC vectors.
	* gcc.target/arm/simd/mve-vcmp-f16.c: New test for
	auto-vectorization.
	* gcc.target/arm/armv8_2-fp16-arith-1.c: Adjust since we now
	vectorize float16_t vectors.