[RISCV] Support Zfa fli instructions with vector splats.

-Return false from RISCVDAGToDAGISel::selectFPImm for fli
 constants so we don't try to use integer expansion.
-Support fli.h with Zvfh+Zfhmin.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D145766