[llvm] [RISCV] Support postRA vsetvl insertion pass (PR #70549)

[Piyou Chen via llvm-commits]

BeMg wrote:

1. Unify the method to retrieve specific VRegDef by getReachingDef. Using getUniqueVRegDef inside getReachingDef because hasNonZeroAVL doesn't exist the reference position as current AVLReg's userMI. 
2. Update the needVSETVLIPHI logic for postRA environment. There are something could improve like PHI node become COPY after Eliminate PHI pass. 

https://github.com/llvm/llvm-project/pull/70549