[GlobalISel] Fix Wint-in-bool-context warning (NFC)

GCC warning:
```
/llvm-project/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp: In member function ‘bool llvm::CombinerHelper::matchFunnelShiftToRotate(llvm::MachineInstr&)’:
/llvm-project/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp:3882:35: warning: ?: using integer constants in boolean context, the expression will always evaluate to ‘true’ [-Wint-in-bool-context]
 3882 |       Opc == TargetOpcode::G_FSHL ? TargetOpcode::G_ROTL : TargetOpcode::G_ROTR;
      |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

@@ -3878,7 +3878,7 @@ bool CombinerHelper::matchFunnelShiftToRotate(MachineInstr &MI) {
   Register Y = MI.getOperand(2).getReg();
   if (X != Y)
     return false;
-  bool RotateOpc =
+  unsigned RotateOpc =
       Opc == TargetOpcode::G_FSHL ? TargetOpcode::G_ROTL : TargetOpcode::G_ROTR;
   return isLegalOrBeforeLegalizer({RotateOpc, {MRI.getType(X), MRI.getType(Y)}});
 }