AArch64: Better codegen for loading from __fp16.

Loading will generally extend to an f32 or an 64, so make sure
to match those patterns directly to load into the FPR16 register
class directly rather than going through the integer GPRs.

This also eliminates an extra step in the convert-to-f64 path
which was first converting to f32 and then to f64 from there.

rdar://17594379