[AArch64][GlobalISel] Select full-fp16 s16 G_FCONSTANT as a constant pool load

When we have full-fp16 support, we should (manually select) s16 G_FCONSTANT to
a constant pool load.

Add support for that to `emitLoadFromConstantPool` + the existing constant
selection code.

Also tidy up the constant selection code a little. There were some out-of-date
comments + some dead code.

Differential Revision: https://reviews.llvm.org/D108957