llvm-0006: PartialInlining IsSingleExit BlockList is_contained O(N^2)

CWE-407: is_contained(BlockList, Succ) inside nested for loops in
computeOutliningColdRegionsInfo. Fix: SmallPtrSet for O(1) membership.
GCC deep scan: CLEAN (bitmaps/hash_sets throughout).

defects/llvm/patch/llvm-0006-partialinlining-IsSingleExit-BlockList-O-N2.patch

@@ -0,0 +1,28 @@
+# UNDF: (leave blank)
+# CWE-407: PartialInlinerImpl IsSingleExit BlockList membership O(N²)
+#
+# In computeOutliningColdRegionsInfo, the IsSingleExit lambda iterates
+# over every block in BlockList and for each successor calls
+# is_contained(BlockList, Succ), which is O(N) per call.
+# Total: O(N × S × N) = O(N² × S) where N = region blocks, S = avg successors.
+#
+# Fix: build a SmallPtrSet from BlockList for O(1) membership lookups.
+#
+# Severity: LOW-MEDIUM — only runs with instrumentation profiling data,
+# but region sizes can be large in profiled builds of complex functions.
+#
+--- a/llvm/lib/Transforms/IPO/PartialInlining.cpp
++++ b/llvm/lib/Transforms/IPO/PartialInlining.cpp
+@@ -368,10 +368,12 @@
+   auto IsSingleExit =
+       [&ORE](SmallVectorImpl<BasicBlock *> &BlockList) -> BasicBlock * {
+     BasicBlock *ExitBlock = nullptr;
++    // Build set for O(1) membership tests (was O(N) linear scan).
++    SmallPtrSet<BasicBlock *, 16> BlockSet(BlockList.begin(), BlockList.end());
+     for (auto *Block : BlockList) {
+       for (BasicBlock *Succ : successors(Block)) {
+-        if (!is_contained(BlockList, Succ)) {
++        if (!BlockSet.count(Succ)) {
+           if (ExitBlock) {
+             ORE.emit([&]() {
+               return OptimizationRemarkMissed(DEBUG_TYPE, "MultiExitRegion",