[AMDGPU] Small cleanup in insertWaitcntInBlock()

[stepthomas]

Summary

Move some code that checks if an instruction is a waitcount into a separate
function, mainly to aid readability in the logic where it is used.

[foad]

LGTM.

llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp

for (int J = 0; J <= VgprUB; J++) {
     unsigned char NewVmemTypes = VgprVmemTypes[J] | Other.VgprVmemTypes[J];
     StrictDom |= NewVmemTypes != VgprVmemTypes[J];
     VgprVmemTypes[J] = NewVmemTypes;
   }
 
   return StrictDom;
 }
 
+static bool isWaitInstr(MachineInstr &Inst) {
+  return Inst.getOpcode() == AMDGPU::S_WAITCNT ||
+         (Inst.getOpcode() == AMDGPU::S_WAITCNT_VSCNT &&
+          Inst.getOperand(0).isReg() &&
+          Inst.getOperand(0).getReg() == AMDGPU::SGPR_NULL);
+}
+
 // Generate s_waitcnt instructions where needed.
 bool SIInsertWaitcnts::insertWaitcntInBlock(MachineFunction &MF,
                                             MachineBasicBlock &Block,
                                             WaitcntBrackets &ScoreBrackets) {
   bool Modified = false;
 
   LLVM_DEBUG({
     dbgs() << "*** Block" << Block.getNumber() << " ***";
bool SIInsertWaitcnts::insertWaitcntInBlock(MachineFunction &MF,
 
   for (MachineBasicBlock::instr_iterator Iter = Block.instr_begin(),
                                          E = Block.instr_end();
        Iter != E;) {
     MachineInstr &Inst = *Iter;
 
     // Track pre-existing waitcnts that were added in earlier iterations or by
     // the memory legalizer.
-    if (Inst.getOpcode() == AMDGPU::S_WAITCNT ||
-        (Inst.getOpcode() == AMDGPU::S_WAITCNT_VSCNT &&
-         Inst.getOperand(0).isReg() &&
-         Inst.getOperand(0).getReg() == AMDGPU::SGPR_NULL)) {
+    if (isWaitInstr(Inst)) {
       if (!OldWaitcntInstr)
         OldWaitcntInstr = &Inst;
       ++Iter;
       continue;
     }
 
     bool FlushVmCnt = Block.getFirstTerminator() == Inst &&
                       isPreheaderToFlush(Block, ScoreBrackets);