[RISCV] Codegen support for Zfhmin.

The Zfhmin subset only has FLH, FSH, FMV.X.H, FMV.H.X, FCVT.S.H, and FCVT.H.S.
If the D extension is present, the FCVT.D.H and FCVT.H.D instructions are also included.
Since most instructions are not included for Zfhmin, so most operations are promoted.
The patch primarily about making f16 a legal type.

RISC-V ISA info:
https://wiki.riscv.org/display/HOME/Recently+Ratified+Extensions

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D139391