[llvm] [BPF] Remove 'may_goto 0' instructions (PR #123482)

[via llvm-commits]

================
@@ -682,6 +685,56 @@ bool BPFMIPreEmitPeephole::insertMissingCallerSavedSpills() {
   return Changed;
 }
 
+bool BPFMIPreEmitPeephole::removeMayGotoZero() {
+  bool Changed = false;
+  MachineBasicBlock *Prev_MBB, *Curr_MBB = nullptr;
+
+  for (MachineBasicBlock &MBB : make_early_inc_range(reverse(*MF))) {
+    Prev_MBB = Curr_MBB;
+    Curr_MBB = &MBB;
+    if (Prev_MBB == nullptr)
+      continue;
+
+    MachineInstr &MI = MBB.back();
+    if (MI.getOpcode() != TargetOpcode::INLINEASM_BR)
+      continue;
+
+    const char *AsmStr = MI.getOperand(0).getSymbolName();
+    SmallVector<StringRef, 4> AsmPieces;
+    SplitString(AsmStr, AsmPieces, ";\n");
----------------
yonghong-song wrote:

I used the approach similar to ARM/ARMISelLowering.cpp:
```
  InlineAsm *IA = cast<InlineAsm>(CI->getCalledOperand());
  StringRef AsmStr = IA->getAsmString();
  SmallVector<StringRef, 4> AsmPieces;
  SplitString(AsmStr, AsmPieces, ";\n");

  switch (AsmPieces.size()) {
  default: return false;
  case 1:
    AsmStr = AsmPieces[0];
    AsmPieces.clear();
    SplitString(AsmStr, AsmPieces, " \t,");

    // rev $0, $1
    if (AsmPieces.size() == 3 &&
        AsmPieces[0] == "rev" && AsmPieces[1] == "$0" && AsmPieces[2] == "$1" &&
        IA->getConstraintString().compare(0, 4, "=l,l") == 0) {
      IntegerType *Ty = dyn_cast<IntegerType>(CI->getType());
      if (Ty && Ty->getBitWidth() == 32)
        return IntrinsicLowering::LowerToByteSwap(CI);
    }
    break;
  }
```
The code already enforced only one asm insn is allowed. see
```
    // Do not support multiple insns in one inline asm.
    if (AsmPieces.size() != 1)
      continue;
```

Maybe I missed something here?

https://github.com/llvm/llvm-project/pull/123482