[llvm] [AMDGPU] Insert before and after instructions that always use GDS (PR #131338)

[Stephen Thomas via llvm-commits]

================
@@ -1982,6 +2003,65 @@ static bool isCacheInvOrWBInst(MachineInstr &Inst) {
          Opc == AMDGPU::GLOBAL_WBINV;
 }
 
+// Return true if the next instruction is S_ENDPGM, following fallthrough
+// blocks if necessary.
+bool SIInsertWaitcnts::isNextENDPGM(MachineBasicBlock::instr_iterator It,
+                                    MachineBasicBlock *Block) const {
+  auto E = Block->instr_end();
+
+  while (true) {
+    if (It == E) {
+      if (auto FallThrough = Block->getFallThrough(false)) {
----------------
stepthomas wrote:

`  auto EndBlock = Block->getParent()->end();

  while (true) {
    if (It.isEnd()) {
      auto NextBlock = std::next(Block->getIterator());
      if (NextBlock != EndBlock) {
        It = NextBlock->instr_begin();
        Block = &*NextBlock;
        continue;
      }

      return false;
    }

    if (!It->isMetaInstruction())
      break;

    It++;
  }

  assert(!It.isEnd());

`

This is what come up with if I want to use `std::next()`. I think it could get a bit more readable if I had an iterator over blocks starting from `Block`, advancing it as the instruction iterator encountered block ends, but is `++` on iterators the same as using `std::next()`?

https://github.com/llvm/llvm-project/pull/131338