AMDGPU: Minor adjustment to r274817

The commit message is inaccurate, modifiesRegister
will check for partial defs of exec.

We currently don't ever emit partial defs of exec,
so it doesn't really matter.

llvm-svn: 274886

llvm/lib/Target/AMDGPU/SILowerControlFlow.cpp

@@ -644,7 +644,7 @@ bool SILowerControlFlow::runOnMachineFunction(MachineFunction &MF) {
       if (TII->isFLAT(MI))
         NeedFlat = true;
 
-      if (I->definesRegister(AMDGPU::EXEC, TRI))
+      if (I->modifiesRegister(AMDGPU::EXEC, TRI))
         ExecModified = true;
 
       switch (MI.getOpcode()) {