[PATCH 10/15][AArch64] Implement vcvt_{,high_}f16_f32

[Alan Lawrence]

gcc/ChangeLog:

        * 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.