[ProfData] Improve efficiency of reader

Pre-reserve space in the map before inserting. In release builds, 9.4%
of all CPU time is spent in llvm::sampleprof::ProfileSymbolList::add. Of
that 9.4%, roughly half is in llvm::DenseMapBase::grow.

Author: mxms

llvm/lib/ProfileData/SampleProf.cpp

@@ -404,9 +404,10 @@ std::error_code ProfileSymbolList::read(const uint8_t *Data,
   uint64_t ExpectedCount = 0;
 
   // Scan forward to see how many elements we expect.
-  while (Offset < ListSize) {
-    if (ListStart[Offset] == '\0') ExpectedCount++;
-    Offset++;
+  while (Size < ListSize) {
+    if (ListStart[Size] == '\0') ExpectedCount++;
+    Size++;
+  }
 
   reserve(ExpectedCount);