[llvm] [RISCV][GISel] Use CCValAssign::getCustomReg for converting f16/f32<->GPR. (PR #105700)

[Craig Topper via llvm-commits]

topperc wrote:

I've remove the new G_ instructions for GISel and tried to match the original code. I feel like Custom is the right way to describe this for SelectionDAG.

There's some additional cleanup that can be done in RISCVISelLowering, but I think its easier to review without adding more changes.

https://github.com/llvm/llvm-project/pull/105700