[RISCV] Add copyPhysRegVector to extract common vector code out of copyPhysRegVector. (#70497)

Call this method directly from each vector case with the correct
arguments. This allows us to treat each type of copy as its own
special case and not pass variables to a common merge point. This
is similar to how AArch64 is structured.
    
I think I can reduce the number of operands to this new method, but
I'll do that as a follow up.