Add a bailout location # threshold in DSE.

In StdlibUnitTest, there is this function that has too many (2450) LSLocations
and the data flow in DSE takes too long to converge.

StdlibUnittest.TestSuite.(addForwardRangeReplaceableCollectionTests <A, B where A: Swift.RangeReplaceableCollectionType, B: Swift.RangeReplaceableCollectionType, A.SubSequence: Swift.CollectionType, B.Generator.Element: Swift.Equatable, A.SubSequence == A.SubSequence.SubSequence, A.Generator.Element == A.SubSequence.Generator.Element> (Swift.String, makeCollection : ([A.Generator.Element]) -> A, wrapValue : (StdlibUnittest.OpaqueValue<Swift.Int>) -> A.Generator.Element, extractValue : (A.Generator.Element) -> StdlibUnittest.OpaqueValue<Swift.Int>, makeCollectionOfEquatable : ([B.Generator.Element]) -> B, wrapValueIntoEquatable : (StdlibUnittest.MinimalEquatableValue) -> B.Generator.Element, extractValueFromEquatable : (B.Generator.Element) -> StdlibUnittest.MinimalEquatableValue, checksAdded : StdlibUnittest.Box<Swift.Set<Swift.String>>, resiliencyChecks : StdlibUnittest.CollectionMisuseResiliencyChecks, outOfBoundsIndexOffset : Swift.Int) -> ()).(closure #18)

This function alone takes ~20% of the total amount of time spent in DSE in StdlibUnitTest.

And DSE does not eliminate any dead store in the function either. I added this threshold
to abort on functions that have too many LSLocations.

I see no difference in # of dead store eliminated in the Stdlib.

Author: trentxintong

lib/SILOptimizer/Transforms/DeadStoreElimination.cpp

@@ -150,6 +150,9 @@ static bool isDeadStoreInertInstruction(SILInstruction *Inst) {
 
 namespace {
 
+// If there are too many locations in the function, we bail out.
+constexpr unsigned MaxLSLocationLimit = 2048;
+
 /// If a large store is broken down to too many smaller stores, bail out.
 /// Currently, we only do partial dead store if we can form a single contiguous
 /// non-dead store.
@@ -1037,6 +1040,10 @@ bool DSEContext::run() {
   // this function.
   LSLocation::enumerateLSLocations(*F, LocationVault, LocToBitIndex, TE);
 
+  // Data flow may take too long to converge.
+  if (LocationVault.size() > MaxLSLocationLimit)
+    return false;
+
   // For all basic blocks in the function, initialize a BB state.
   //
   // DenseMap has a minimum size of 64, while many functions do not have more