PredictableMemOpt: Don't try to diagnose recursive value types.

They're invalid, and we don't want to blow the stack trying to decompose a type with infinite elements. Fixes rdar://problem/17920535.

Swift SVN r23775

lib/SILPasses/PredictableMemOpt.cpp

@@ -31,6 +31,9 @@ STATISTIC(NumAllocRemoved, "Number of allocations completely removed");
 //===----------------------------------------------------------------------===//
 
 static unsigned getNumSubElements(SILType T, SILModule &M) {
+  if (!M.getTypeLowering(T).isValid())
+    return 0;
+
   if (auto TT = T.getAs<TupleType>()) {
     unsigned NumElements = 0;
     for (auto index : indices(TT.getElementTypes()))