[GlobalISel] Move getOpcode() calls inside assert() to avoid (void)s. NFC.

Tidier solution to the unused variable warnings - we already do this in other places in this file.

Author: RKSimon

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

@@ -4618,9 +4618,8 @@ bool CombinerHelper::matchMulOBy2(MachineInstr &MI, BuildFnTy &MatchInfo) {
 
 bool CombinerHelper::matchMulOBy0(MachineInstr &MI, BuildFnTy &MatchInfo) {
   // (G_*MULO x, 0) -> 0 + no carry out
-  unsigned Opc = MI.getOpcode();
-  (void)Opc;
-  assert(Opc == TargetOpcode::G_UMULO || Opc == TargetOpcode::G_SMULO);
+  assert(MI.getOpcode() == TargetOpcode::G_UMULO ||
+         MI.getOpcode() == TargetOpcode::G_SMULO);
   if (!mi_match(MI.getOperand(3).getReg(), MRI, m_SpecificICstOrSplat(0)))
     return false;
   Register Dst = MI.getOperand(0).getReg();
@@ -4637,9 +4636,8 @@ bool CombinerHelper::matchMulOBy0(MachineInstr &MI, BuildFnTy &MatchInfo) {
 
 bool CombinerHelper::matchAddOBy0(MachineInstr &MI, BuildFnTy &MatchInfo) {
   // (G_*ADDO x, 0) -> x + no carry out
-  unsigned Opc = MI.getOpcode();
-  (void)Opc;
-  assert(Opc == TargetOpcode::G_UADDO || Opc == TargetOpcode::G_SADDO);
+  assert(MI.getOpcode() == TargetOpcode::G_UADDO ||
+         MI.getOpcode() == TargetOpcode::G_SADDO);
   if (!mi_match(MI.getOperand(3).getReg(), MRI, m_SpecificICstOrSplat(0)))
     return false;
   Register Carry = MI.getOperand(1).getReg();