AMDGPU: Cleanup. Use definesRegister instead of manual loop

[arsenm]

Summary

Also this will be more precise since it will check exec_lo/exec_hi writes.

[nhaehnle]

LGTM

[arsenm]

r274817

lib/Target/AMDGPU/SILowerControlFlow.cpp

for (I = MBB.begin(); I != MBB.end(); I = Next) {
       Next = std::next(I);
 
       MachineInstr &MI = *I;
 
       // Flat uses m0 in case it needs to access LDS.
       if (TII->isFLAT(MI))
         NeedFlat = true;
 
-      for (const auto &Def : I->defs()) {
-        if (Def.isReg() && Def.isDef() && Def.getReg() == AMDGPU::EXEC) {
-          ExecModified = true;
-          break;
-        }
-      }
+      if (I->definesRegister(AMDGPU::EXEC, TRI))
 
       switch (MI.getOpcode()) {
         default: break;
         case AMDGPU::SI_IF:
           ++Depth;
           If(MI);
           break;