[ARM NEON] Use the common naming scheme for vcvt f16 builtins. NFC.

We had "vcvt_f16" and "VCVT_HIGH_F16": for other FP types, this naming
is used for intrinsics with integer overloads. The FP->FP conversions,
on the other hand, use the full "vcvt_f32_f64" name instead.

Use the same naming convention for the f16<->f32 conversions.
While there, reorder the definitions a little bit.

llvm-svn: 245763