Fix CFG building issue.

Summary:
Fixed getBasicBlockContainingOffset() to return correct basic
block.

(cherry picked from FBD2532514)

bolt/BinaryFunction.cpp

@@ -31,6 +31,22 @@ namespace llvm {
 
 namespace flo {
 
+BinaryBasicBlock *
+BinaryFunction::getBasicBlockContainingOffset(uint64_t Offset) {
+  if (Offset > Size)
+    return nullptr;
+
+  if (BasicBlocks.empty())
+    return nullptr;
+
+  auto I = std::upper_bound(BasicBlocks.begin(),
+                            BasicBlocks.end(),
+                            BinaryBasicBlock(Offset));
+  assert(I != BasicBlocks.begin() && "first basic block not at offset 0");
+
+  return &(*--I);
+}
+
 void BinaryFunction::print(raw_ostream &OS, bool PrintInstructions) const {
   StringRef SectionName;
   Section.getName(SectionName);

bolt/BinaryFunction.h

@@ -263,22 +263,7 @@ public:
 
   /// Return basic block that originally contained offset \p Offset
   /// from the function start.
-  BinaryBasicBlock *getBasicBlockContainingOffset(uint64_t Offset) {
-    if (Offset > Size)
-      return nullptr;
-
-    if (BasicBlocks.empty())
-      return nullptr;
-
-    auto I = std::lower_bound(BasicBlocks.begin(),
-                              BasicBlocks.end(),
-                              BinaryBasicBlock(Offset));
-
-    if (I == BasicBlocks.end())
-      return &BasicBlocks.back();
-
-    return &(*I);
-  }
+  BinaryBasicBlock *getBasicBlockContainingOffset(uint64_t Offset);
 
   /// Dump function information to debug output. If \p PrintInstructions
   /// is true - include instruction disassembly.