Change an assert() to a diagnostic.
AArch64: Better codegen for storing to __fp16.

Storing will generally be immediately preceded by rounding from an f32
or f64, so make sure to match those patterns directly to convert into the
FPR16 register class directly rather than going through the integer GPRs.

This also eliminates an extra step in the convert-from-f64 path
which was first converting to f32 and then to f16 from there.

rdar://17594379