[RISCV][NFC] Refactor LocVT<->ValVT converstion in RISCVISelLowering

There was some duplicated logic for using the LocInfo of a CCValAssign in 
order to convert from the ValVT to LocVT or vice versa. Resolve this by 
factoring out convertLocVTFromValVT from unpackFromRegLoc. Also rename 
packIntoRegLoc to the more appropriate convertValVTToLocVT and call these 
helper functions consistently.

llvm-svn: 343737