[llvm] Ensure NoTrapAfterNoreturn is false for the wasm backend (PR #65876)

[Sam Clegg via llvm-commits]

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@@ -109,6 +110,53 @@ static bool maybeRewriteToFallthrough(MachineInstr &MI, MachineBasicBlock &MBB,
   return true;
 }
 
+static bool eraseDeadCodeAroundUnreachable(MachineInstr &UnreachbleMI, MachineBasicBlock &MBB) {
+  SmallVector<MachineInstr*, 16> ToDelete;
+
+  // Because wasm unreachable is stack polymorphic and unconditionally ends control,
+  // all instructions after it can be removed until the end of this block.
+  // We remove the common case of double unreachable.
+  auto ForwardsIterator = UnreachbleMI.getIterator();
+  for (ForwardsIterator++; !ForwardsIterator.isEnd(); ForwardsIterator++) {
+    MachineInstr& MI = *ForwardsIterator;
+    if (MI.getOpcode() == WebAssembly::UNREACHABLE) {
+      ToDelete.push_back(&MI);
+    } else {
+      break;
+    }
+  }
+
+  [&]() {
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sbc100 wrote:

Oh neat trick.  Maybe a bit too clever, or a pattern that I'm just not used to seeing in the wild.   `goto` sounds better to me but if folks disagree then at least a comment would be good. 

https://github.com/llvm/llvm-project/pull/65876