[RISCV] Enable Zfa fli.h instruction in MC layer with Zfhmin and Zvfh.

According to the spec this instruction is can be enabled with Zfh
and Zvfh (which requires Zfhmin). The other instructions f16
instructions from Zfa require Zfh.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D145649