[RISCV] Use empty() instead of size()==0. NFC (#149868)

Move the assert past the code that determines if the pass should run.

Author: topperc

llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp

@@ -1486,7 +1486,6 @@ bool RISCVVLOptimizer::tryReduceVL(MachineInstr &MI) const {
 }
 
 bool RISCVVLOptimizer::runOnMachineFunction(MachineFunction &MF) {
-  assert(DemandedVLs.size() == 0);
   if (skipFunction(MF.getFunction()))
     return false;
 
@@ -1499,6 +1498,8 @@ bool RISCVVLOptimizer::runOnMachineFunction(MachineFunction &MF) {
 
   TII = ST.getInstrInfo();
 
+  assert(DemandedVLs.empty());
+
   // For each instruction that defines a vector, compute what VL its
   // downstream users demand.
   for (MachineBasicBlock *MBB : post_order(&MF)) {