[CodeGen] When Promoting, don't extend the 2nd FCOPYSIGN operand.

We don't care about its type, and there's even a combine that'll fold
away the FP_EXTEND if we let it run. However, until it does, we'll have
something broken like:
  (f32 (fp_extend (f64 v)))

Scalar f16 follow-up to r243924.

llvm-svn: 244858