[RISCV] Add LiveInterval assertions in insertVSETVLI. NFC

If we're inserting a vsetvli that uses a register AVL, then the AVL
register should either:

a) Be already live at the vsetvli with the expected value
b) Not be live at the vsetvli, but have exactly one value that can be
   extended safely: see #97264

Author: lukel97

llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp

@@ -1163,7 +1163,10 @@ void RISCVInsertVSETVLI::insertVSETVLI(MachineBasicBlock &MBB,
     // isn't always the case, e.g. PseudoVMV_X_S doesn't have an AVL operand or
     // we've taken the AVL from the VL output of another vsetvli.
     LiveInterval &LI = LIS->getInterval(AVLReg);
-    LIS->extendToIndices(LI, {LIS->getInstructionIndex(*MI).getRegSlot()});
+    SlotIndex SI = LIS->getInstructionIndex(*MI).getRegSlot();
+    assert((LI.liveAt(SI) && LI.getVNInfoAt(SI) == Info.getAVLVNInfo()) ||
+           (!LI.liveAt(SI) && LI.containsOneValue()));
+    LIS->extendToIndices(LI, SI);
   }
 }