[polly][Support] Un-break polly tests

Previously, the polly unit tests were stuck in a infinite loop.
There was an edge case in StringRef::count() introduced by 9f6b13e, where an empty 'Str' would cause the function to never exit.
Also fixed usage in polly.

Author: aganea

llvm/lib/Support/StringRef.cpp

@@ -372,7 +372,7 @@ void StringRef::split(SmallVectorImpl<StringRef> &A, char Separator,
 size_t StringRef::count(StringRef Str) const {
   size_t Count = 0;
   size_t N = Str.size();
-  if (N > Length)
+  if (!N || N > Length)
     return 0;
   for (size_t i = 0, e = Length - N + 1; i < e;) {
     if (substr(i, N).equals(Str)) {

llvm/unittests/ADT/StringRefTest.cpp

@@ -509,6 +509,7 @@ TEST(StringRefTest, Count) {
   EXPECT_EQ(1U, Str.count("hello"));
   EXPECT_EQ(1U, Str.count("ello"));
   EXPECT_EQ(0U, Str.count("zz"));
+  EXPECT_EQ(0U, Str.count(""));
 
   StringRef OverlappingAbba("abbabba");
   EXPECT_EQ(1U, OverlappingAbba.count("abba"));

polly/lib/Analysis/ScopDetection.cpp

@@ -1653,7 +1653,8 @@ bool ScopDetection::isValidRegion(DetectionContext &Context) const {
                                             CurRegion.getExit(), DbgLoc);
   }
 
-  if (!CurRegion.getEntry()->getName().count(OnlyRegion)) {
+  if (!OnlyRegion.empty() &&
+      !CurRegion.getEntry()->getName().count(OnlyRegion)) {
     LLVM_DEBUG({
       dbgs() << "Region entry does not match -polly-region-only";
       dbgs() << "\n";