[llvm] [RISCV] Teach RISCVInsertVSETVLI to work without LiveIntervals (PR #94686)

[Luke Lau via llvm-commits]

================
@@ -52,6 +52,8 @@ namespace {
 static VNInfo *getVNInfoFromReg(Register Reg, const MachineInstr &MI,
----------------
lukel97 wrote:

Nit, should probably update the comment to mention it will return nullptr if LIS is null too

https://github.com/llvm/llvm-project/pull/94686