[CodeGen] Don't assume fp_to_fp16 produces i16 when legalizing it.

Since r230276, we support an improved legalization for f64->f16,
which goes through a temporary f32, improving codegen when
f32->f16 is legal but not f64->f16. This requires unsafe-fp-math.

However, that legalization assumed that the second step, producing
a pseudo-softened f16, had type i16. That's not true on targets
with illegal i16, such as ARM.

Use the initial f64->f16 result type instead.

llvm-svn: 257794