[CSSPGO] Turn on iterative-BFI for CSSPGO

Iterative-BFI produces better count quality and performance when evaluated on internal benchmarks. Turning it on by default now for CSSPGO. We can consider turn it on by default for AutoFDO as well in the future.

Differential Revision: https://reviews.llvm.org/D106202

llvm/lib/Transforms/IPO/SampleProfile.cpp

@@ -36,6 +36,7 @@
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/Analysis/AssumptionCache.h"
+#include "llvm/Analysis/BlockFrequencyInfoImpl.h"
 #include "llvm/Analysis/CallGraph.h"
 #include "llvm/Analysis/CallGraphSCCPass.h"
 #include "llvm/Analysis/InlineAdvisor.h"
@@ -1804,6 +1805,10 @@ bool SampleProfileLoader::doInitialization(Module &M,
     if (!CallsitePrioritizedInline.getNumOccurrences())
       CallsitePrioritizedInline = true;
 
+    // Enable iterative-BFI by default for CSSPGO.
+    if (!UseIterativeBFIInference.getNumOccurrences())
+      UseIterativeBFIInference = true;
+
     // Tracker for profiles under different context
     ContextTracker =
         std::make_unique<SampleContextTracker>(Reader->getProfiles());