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

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