[llvm] [RISCV] Add 16 bit GPR sub-register for Zhinx. (PR #107446)

[Philip Reames via llvm-commits]

preames wrote:

> FLH and LH both use half as the term, and the format of the float shouldn’t affect the load or store, just the interpretation for arithmetic purposes, so _H seems like the appropriate thing to use for me?

Good point, naming suggestion withdrawn.  

https://github.com/llvm/llvm-project/pull/107446