[AArch64] Implement vcvt_{,high_}f16_f32

[AArch64] Implement vcvt_{,high_}f16_f32

	* config/aarch64/aarch64-simd.md (aarch64_float_truncate_lo_v2sf):
	Reparameterize to...
	(aarch64_float_truncate_lo_<mode>): ...this, for both V2SF and V4HF.
	(aarch64_float_truncate_hi_v4sf): Reparameterize to...
	(aarch64_float_truncate_hi_<Vdbl>): ...this, for both V4SF and V8HF.

	* config/aarch64/aarch64-simd-builtins.def (float_truncate_hi_): Add
	v8hf variant.
	(float_truncate_lo_): Use BUILTIN_VDF iterator.

	* config/aarch64/arm_neon.h (vcvt_f16_f32, vcvt_high_f16_f32): New.

	* config/aarch64/iterators.md (VDF, Vdtype): New.
	(VWIDE, Vmwtype): Add cases for V4HF and V2SF.

From-SVN: r227545