[AArch64] Remove an unnecessary cast (NFC) (llvm#156139)

getOpcode() already returns unsigned.

Author: kazutakahirata

llvm/lib/Target/AArch64/AArch64InstrInfo.cpp

@@ -6718,7 +6718,7 @@ static bool canCombine(MachineBasicBlock &MBB, MachineOperand &MO,
   if (MO.isReg() && MO.getReg().isVirtual())
     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)
+  if (!MI || MI->getParent() != &MBB || MI->getOpcode() != CombineOpc)
     return false;
   // Must only used by the user we combine with.
   if (!MRI.hasOneNonDBGUse(MI->getOperand(0).getReg()))