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

[Piyou Chen via llvm-commits]

BeMg wrote:


Rebase with the lastest main branch because patch raise a lot of conflicts.

Also 

1. Make getReachingDefMI accept `const MachineInstr*` and `MachineInstr*`
2. Make canMutatePriorConfig handle non-SSA situation
3. Use fixupModifyVRegLIAfterInsertMI and removeMIAndFixupModifyVRegLI to make LiveInterval fixup more clear.
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@lukel97 @preames @topperc @kito-cheng ping

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