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().

llvm-svn: 237611
diff --git a/llvm/lib/CodeGen/MachineVerifier.cpp b/llvm/lib/CodeGen/MachineVerifier.cpp
index ec85e55..f5edcb7 100644
--- a/llvm/lib/CodeGen/MachineVerifier.cpp
+++ b/llvm/lib/CodeGen/MachineVerifier.cpp
@@ -1716,8 +1716,8 @@
 /// by a FrameDestroy <n>, stack adjustments are identical on all
 /// CFG edges to a merge point, and frame is destroyed at end of a return block.
 void MachineVerifier::verifyStackFrame() {
-  int FrameSetupOpcode   = TII->getCallFrameSetupOpcode();
-  int FrameDestroyOpcode = TII->getCallFrameDestroyOpcode();
+  unsigned FrameSetupOpcode   = TII->getCallFrameSetupOpcode();
+  unsigned FrameDestroyOpcode = TII->getCallFrameDestroyOpcode();
 
   SmallVector<StackStateOfBB, 8> SPState;
   SPState.resize(MF->getNumBlockIDs());