[AMDGPU] SIInsertSkips: Refactor early exit block creation

[critson]

Summary

Refactor exit block creation to a single call ensureEarlyExitBlock.
Add support for generating an early exit block which clears the
exec mask, but only add this instruction when required.
These changes are to facilitate adding more forms of early
termination for PS shaders in the near future.

[critson]

Remove more unused code.

[critson]

Add missing variable reset.

[nhaehnle]

LGTM

llvm/lib/Target/AMDGPU/SIInsertSkips.cpp

 class SIInsertSkips : public MachineFunctionPass {
 private:
   const SIRegisterInfo *TRI = nullptr;
   const SIInstrInfo *TII = nullptr;
   unsigned SkipThreshold = 0;
   MachineDominatorTree *MDT = nullptr;
 
   MachineBasicBlock *EarlyExitBlock = nullptr;
+  bool EarlyExitClearsExec = false;
 
   bool shouldSkip(const MachineBasicBlock &From,
                   const MachineBasicBlock &To) const;
 
   bool dominatesAllReachable(MachineBasicBlock &MBB);
-  void createEarlyExitBlock(MachineBasicBlock &MBB);
+  void ensureEarlyExitBlock(MachineBasicBlock &MBB, bool ClearExec);
   void skipIfDead(MachineBasicBlock &MBB, MachineBasicBlock::iterator I,
                   DebugLoc DL);
 
   bool kill(MachineInstr &MI);
 
   bool skipMaskBranch(MachineInstr &MI, MachineBasicBlock &MBB);
 
 public:
BuildMI(MBB, I, DL, TII->get(AMDGPU::EXP_DONE))
       .addReg(AMDGPU::VGPR0, RegState::Undef)
       .addReg(AMDGPU::VGPR0, RegState::Undef)
       .addImm(1)  // vm
       .addImm(0)  // compr
       .addImm(0); // en
   BuildMI(MBB, I, DL, TII->get(AMDGPU::S_ENDPGM)).addImm(0);
 }
 
-void SIInsertSkips::createEarlyExitBlock(MachineBasicBlock &MBB) {
+void SIInsertSkips::ensureEarlyExitBlock(MachineBasicBlock &MBB,
+                                         bool ClearExec) {
   MachineFunction *MF = MBB.getParent();
   DebugLoc DL;
 
-  assert(!EarlyExitBlock);
+  if (!EarlyExitBlock) {
     EarlyExitBlock = MF->CreateMachineBasicBlock();
     MF->insert(MF->end(), EarlyExitBlock);
-
     generatePsEndPgm(*EarlyExitBlock, EarlyExitBlock->end(), DL, TII);
+    EarlyExitClearsExec = false;
+  }
+
+  if (ClearExec && !EarlyExitClearsExec) {
+    const GCNSubtarget &ST = MF->getSubtarget<GCNSubtarget>();
+    unsigned Mov = ST.isWave32() ? AMDGPU::S_MOV_B32 : AMDGPU::S_MOV_B64;
+    Register Exec = ST.isWave32() ? AMDGPU::EXEC_LO : AMDGPU::EXEC;
+    auto ExitI = EarlyExitBlock->getFirstNonPHI();
+    assert(ExitI->getOpcode() == AMDGPU::EXP_DONE);
+    BuildMI(*EarlyExitBlock, ExitI, DL, TII->get(Mov), Exec).addImm(0);
+    EarlyExitClearsExec = true;
+  }
 }
 
 static void splitBlock(MachineBasicBlock &MBB, MachineInstr &MI,
                        MachineDominatorTree *MDT) {
   MachineBasicBlock *SplitBB = MBB.splitAt(MI, /*UpdateLiveIns*/ true);
 
   // Update dominator tree
   using DomTreeT = DomTreeBase<MachineBasicBlock>;
void SIInsertSkips::skipIfDead(MachineBasicBlock &MBB,
   // already-existing basic block.
   auto NextBBI = std::next(MBB.getIterator());
   bool NoSuccessor = I == MBB.end() &&
                      llvm::find(MBB.successors(), &*NextBBI) == MBB.succ_end();
 
   if (NoSuccessor) {
     generatePsEndPgm(MBB, I, DL, TII);
   } else {
-    if (!EarlyExitBlock) {
-      createEarlyExitBlock(MBB);
-      // Update next block pointer to reflect any new blocks
-      NextBBI = std::next(MBB.getIterator());
-    }
+    ensureEarlyExitBlock(MBB, false);
 
     MachineInstr *BranchMI =
         BuildMI(MBB, I, DL, TII->get(AMDGPU::S_CBRANCH_EXECZ))
             .addMBB(EarlyExitBlock);
 
     // Split the block if the branch will not come at the end.
     auto Next = std::next(BranchMI->getIterator());
     if (Next != MBB.end() && !Next->isTerminator())