AMDGPU: Remove register operand from si_mask_branch

[arsenm]

Summary

It isn't used for anything, and is also misleading since
it could be spilled at the end of the block, so it can't be relied
on. There ends up being a verifier error about using an undefined
register since the spill kills the register.

[arsenm]

r279899

lib/Target/AMDGPU/SIInstructions.td

 // SI pseudo instructions. These are used by the CFG structurizer pass
 // and should be lowered to ISA instructions prior to codegen.
 
 let hasSideEffects = 1 in {
 
 // Dummy terminator instruction to use after control flow instructions
 // replaced with exec mask operations.
 def SI_MASK_BRANCH : PseudoInstSI <
-  (outs), (ins brtarget:$target, SReg_64:$dst)> {
+  (outs), (ins brtarget:$target)> {
   let isBranch = 0;
   let isTerminator = 1;
   let isBarrier = 0;
   let SALU = 1;
   let Uses = [EXEC];
 }
 
 let Uses = [EXEC], Defs = [EXEC, SCC] in {

lib/Target/AMDGPU/SILowerControlFlow.cpp

MachineInstr *Xor =
     BuildMI(MBB, I, DL, TII->get(AMDGPU::S_XOR_B64), SaveExecReg)
     .addReg(AMDGPU::EXEC)
     .addReg(SaveExecReg);
 
   // Insert a pseudo terminator to help keep the verifier happy. This will also
   // be used later when inserting skips.
   MachineInstr *NewBr =
     BuildMI(MBB, I, DL, TII->get(AMDGPU::SI_MASK_BRANCH))
-    .addOperand(MI.getOperand(2))
-    .addReg(SaveExecReg, getKillRegState(SaveExec.isKill()));
+    .addOperand(MI.getOperand(2));
 
   if (!LIS) {
     MI.eraseFromParent();
     return;
   }
 
 
   LIS->ReplaceMachineInstrInMaps(MI, *AndSaveExec);
MachineInstr *Xor =
     BuildMI(MBB, ElsePt, DL, TII->get(AMDGPU::S_XOR_B64), AMDGPU::EXEC)
     .addReg(AMDGPU::EXEC)
     .addReg(DstReg);
 
   MachineBasicBlock::iterator Term = MBB.getFirstTerminator();
   // Insert a pseudo terminator to help keep the verifier happy.
   MachineInstr *Branch =
     BuildMI(MBB, Term, DL, TII->get(AMDGPU::SI_MASK_BRANCH))
-    .addMBB(DestBB)
-    .addReg(DstReg);
+    .addMBB(DestBB);
 
   if (!LIS) {
     MI.eraseFromParent();
     return;
   }
 
   LIS->RemoveMachineInstrFromMaps(MI);
   MI.eraseFromParent();