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

[Tim Northover]

> Sort-of. It’s not wrong to go through float, but doing so does require care about rounding (which we weren’t doing).

And vice-versa, surely? If we were asked in the IR to perform a double
-> float -> fp16 conversion don't we have to do that unless we know
the result isn't changed?

Which is why I think this double-conversion has to be fixed in Clang:
the input IR *is* saying to do it in two steps.

Cheers.

Tim.