[llvm] [CodeGen] Fix lpad padding at section start after empty block (PR #112595)

[Daniel Hoekwater via llvm-commits]

================
@@ -265,8 +265,22 @@ void llvm::sortBasicBlocksAndUpdateBranches(
 // zero implies "no landing pad." This function inserts a NOP just before the EH
 // pad label to ensure a nonzero offset.
 void llvm::avoidZeroOffsetLandingPad(MachineFunction &MF) {
+  std::optional<MBBSectionID> CurrentSection;
+  bool SectionEmpty = true;
+  auto IsFirstNonEmptyBBInSection = [&](const MachineBasicBlock &MBB) {
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dhoekwater wrote:

This function may give false positives in the following situation:
```
bb0 (bbsections 0)
bb1 (bbsections cold)
bb2 (bbsections 0)
```

If false positives are fine (since we're just inserting a `nop`), then you can disregard this feedback. Otherwise, you may want to do something like
```
llvm::DenseSet<MBBSectionID> SectionsWithNonEmptyBlocks;
auto IsFirstNonEmptyBBInSection = [&](const MachineBasicBlock &MBB) {
  if (MBB.empty() || SectionsWithNonEmptyBlocks.contains(MBB.getSectionID()))
    return false;

  SectionsWithNonEmptyBlocks.insert(MBB.getSectionID());
```

https://github.com/llvm/llvm-project/pull/112595