[RISCV] Separate FPR and VR copyPhysReg implementation. (#70492)

This duplicates the BuildMI from the 3 FP copy types, but separates them
from the VR code and gets rid of the IsScalableVector flag.

I need to add FPR<->GPR copies for GISel which needs another variation
of BuildMI. So it seems cleaner to start enumerating each case as its
own if.