[RISCV] Separate the Zfhmin and Zfh extensions.

The spec doesn't seem to be written as if Zfh implies Zfhmin. They
seem to be separate extensions.

This patch moves the instructions from Zfhmin to be enabled with
either the Zfh or Zfhmin extensions.

Reviewed By: achieveartificialintelligence

Differential Revision: https://reviews.llvm.org/D118581