[AArch64][GlobalISel] Mark G_FCONSTANT as legal when there is full fp16 support

When there is full fp16 support, there is no reason to widen 16-bit
G_FCONSTANTs to 32 bits. Mark them as legal in this case.

Also, we currently import a pattern for materializing a 16-bit 0.0.
Add a testcase showing we select it.

(All other 16-bit G_FCONSTANTS are not yet selected.)

Differential Revision: https://reviews.llvm.org/D89164