[CodeGen] Remove an unnecessary cast (NFC) (llvm#151901)

getOpcode() already returns unsigned.

Author: kazutakahirata

llvm/lib/CodeGen/TargetInstrInfo.cpp

@@ -993,7 +993,7 @@ static bool canCombine(MachineBasicBlock &MBB, MachineOperand &MO,
     MI = MRI.getUniqueVRegDef(MO.getReg());
   // And it needs to be in the trace (otherwise, it won't have a depth).
   if (!MI || MI->getParent() != &MBB ||
-      ((unsigned)MI->getOpcode() != CombineOpc && CombineOpc != 0))
+      (MI->getOpcode() != CombineOpc && CombineOpc != 0))
     return false;
   // Must only used by the user we combine with.
   if (!MRI.hasOneNonDBGUse(MI->getOperand(0).getReg()))