MachineInstr: Change return value of getOpcode() to unsigned.

This was previously returning int. However there are no negative opcode
numbers and more importantly this was needlessly different from
MCInstrDesc::getOpcode() (which even is the value returned here) and
SDValue::getOpcode()/SDNode::getOpcode().

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237611 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/SystemZ/SystemZInstrInfo.cpp

@@ -423,7 +423,7 @@ static MachineInstr *getDef(unsigned Reg,
 }
 
 // Return true if MI is a shift of type Opcode by Imm bits.
-static bool isShift(MachineInstr *MI, int Opcode, int64_t Imm) {
+static bool isShift(MachineInstr *MI, unsigned Opcode, int64_t Imm) {
   return (MI->getOpcode() == Opcode &&
           !MI->getOperand(2).getReg() &&
           MI->getOperand(3).getImm() == Imm);