[llvm] [RISCV][llvm] Refactor unpackFromMemLoc to use convertLocVTToValVT (PR #175969)

[Brandon Wu via llvm-commits]

4vtomat wrote:

> Is this NFC? If so, put it in the title

I think not quite? `convertLocVTToValVT` perform bitcast if locinfo is `CCValAssign::BCvt` that we didn't handle because there's no such case for now, but we are going to have one when supporting fixed vector on P extension

https://github.com/llvm/llvm-project/pull/175969