AMDGPU/SI: Fix missing implicit def/use of exec

[arsenm]

Summary

Not sure why this wasn't caught by the machineinstr verifier.

[arsenm]

Abandoning. These are supposed to be added by the initial machine instruction creation and these are verified

lib/Target/AMDGPU/SILowerControlFlow.cpp

#endif
   if ((Op.isImm())) {
     // Constant operand: Set exec mask to 0 or do nothing
     if (Op.getImm() & 0x80000000) {
       BuildMI(MBB, &MI, DL, TII->get(AMDGPU::S_MOV_B64), AMDGPU::EXEC)
               .addImm(0);
     }
   } else {
     BuildMI(MBB, &MI, DL, TII->get(AMDGPU::V_CMPX_LE_F32_e32), AMDGPU::VCC)
-           .addImm(0)
-           .addOperand(Op);
+      .addOperand(Op)
+      .addReg(AMDGPU::EXEC, RegState::Implicit | RegState::Define)
+      .addReg(AMDGPU::EXEC, RegState::Implicit);
   }
 
   MI.eraseFromParent();
 }
 
 void SILowerControlFlowPass::LoadM0(MachineInstr &MI, MachineInstr *MovRel, int Offset) {
 
   MachineBasicBlock &MBB = *MI.getParent();