[MLGO][NFC] Use std::map instead of DenseMap to avoid use after free

In `MLInlineAdvisor::getAdviceImpl`, we call `getCachedFPI` twice, once
for the caller, once for the callee, so the second may invalidate the
reference obtained by the first because the underlying implementation of
the cache is a `DenseMap`. `std::map` doesn't have that problem.

Author: mtrofin

llvm/include/llvm/Analysis/MLInlineAdvisor.h

@@ -69,7 +69,7 @@ class MLInlineAdvisor : public InlineAdvisor {
   getSkipAdviceIfUnreachableCallsite(CallBase &CB);
   void print(raw_ostream &OS) const override;
 
-  mutable DenseMap<const Function *, FunctionPropertiesInfo> FPICache;
+  mutable std::map<const Function *, FunctionPropertiesInfo> FPICache;
 
   LazyCallGraph &CG;
 

llvm/lib/Analysis/MLInlineAdvisor.cpp

@@ -415,8 +415,8 @@ void MLInlineAdvisor::print(raw_ostream &OS) const {
      << " EdgesOfLastSeenNodes: " << EdgesOfLastSeenNodes << "\n";
   OS << "[MLInlineAdvisor] FPI:\n";
   for (auto I : FPICache) {
-    OS << I.getFirst()->getName() << ":\n";
-    I.getSecond().print(OS);
+    OS << I.first->getName() << ":\n";
+    I.second.print(OS);
     OS << "\n";
   }
   OS << "\n";