[SILOptimizer]: slow OSSA lifetime canonicalization mitigation

OSSA lifetime canonicalization can take a very long time in certain
cases in which there are large basic blocks. to mitigate this, add logic
to skip walking the liveness boundary for extending liveness extension
to dead ends when there aren't any dead ends in the function.

Author: jamieQ

lib/SILOptimizer/Utils/CanonicalizeOSSALifetime.cpp

@@ -125,6 +125,19 @@ static bool isDestroyOfCopyOf(SILInstruction *instruction, SILValue def) {
   return false;
 }
 
+static bool
+functionHasAnyDeadEnds(SILFunction *function,
+                       DeadEndBlocksAnalysis *deadEndBlocksAnalysis) {
+  bool hasAnyDeadEnds = false;
+  auto *deadEnds = deadEndBlocksAnalysis->get(function);
+  for (auto &bb : *function) {
+    hasAnyDeadEnds = deadEnds->isDeadEnd(&bb);
+    if (hasAnyDeadEnds)
+      break;
+  }
+  return hasAnyDeadEnds;
+}
+
 //===----------------------------------------------------------------------===//
 // MARK: Step 1. Compute pruned liveness
 //===----------------------------------------------------------------------===//
@@ -300,6 +313,10 @@ void CanonicalizeOSSALifetime::extendLexicalLivenessToDeadEnds() {
   SSAPrunedLiveness directLiveness(function, &directDiscoverdBlocks);
   directLiveness.initializeDef(getCurrentDef());
   directLiveness.computeSimple();
+
+  if (!functionHasAnyDeadEnds(function, deadEndBlocksAnalysis))
+    return;
+
   OSSALifetimeCompletion::visitAvailabilityBoundary(
       getCurrentDef(), directLiveness, [&](auto *unreachable, auto end) {
         if (end == OSSALifetimeCompletion::LifetimeEnd::Boundary) {
@@ -373,6 +390,9 @@ void CanonicalizeOSSALifetime::extendLivenessToDeadEnds() {
     completeLiveness.updateForUse(pair.first, /*lifetimeEnding=*/false);
   }
 
+  if (!functionHasAnyDeadEnds(function, deadEndBlocksAnalysis))
+    return;
+
   OSSALifetimeCompletion::visitAvailabilityBoundary(
       getCurrentDef(), completeLiveness, [&](auto *unreachable, auto end) {
         if (end == OSSALifetimeCompletion::LifetimeEnd::Boundary) {