[llvm] r212638 - AArch64: Better codegen for storing to __fp16.

[Tim Northover]

> (*) As Tim and I discussed off-list yesterday, __fp16 isn’t mentioned
> anywhere by C / C++, so there isn’t a strong standards requirement
> here.  However, in order for the type to be useful, we should attempt
> to have __fp16 conform to the IEEE-754 rules for the “binary16" interchange
> type, which includes getting these conversions “right” (i.e. conforming
> to the usual rules for conversions that apply to all FP types).

Yep. I definitely agree that if "(double)*my_fp16" means anything, it
should be a single-step conversion. Which means we should fix Clang's
CodeGen.

Cheers.

Tim.