[BOLT] Fix warnings

This patch fixes:

  bolt/lib/Core/BinaryFunctionProfile.cpp:222:10: error: variable
  'BBMergeSI' set but not used [-Werror,-Wunused-but-set-variable]

  bolt/lib/Passes/VeneerElimination.cpp:67:12: error: variable
  'VeneerCallers' set but not used [-Werror,-Wunused-but-set-variable]

Author: kazutakahirata

bolt/lib/Core/BinaryFunctionProfile.cpp

@@ -225,6 +225,7 @@ void BinaryFunction::mergeProfileDataInto(BinaryFunction &BF) const {
     for (const BinaryBasicBlock *BBSucc : BB->successors()) {
       (void)BBSucc;
       assert(getIndex(BBSucc) == BF.getIndex(*BBMergeSI));
+      (void)BBMergeSI;
 
       // At this point no branch count should be set to COUNT_NO_PROFILE.
       assert(BII->Count != BinaryBasicBlock::COUNT_NO_PROFILE &&

bolt/lib/Passes/VeneerElimination.cpp

@@ -89,6 +89,7 @@ void VeneerElimination::runOnFunctions(BinaryContext &BC) {
   LLVM_DEBUG(
       dbgs() << "BOLT-INFO: number of linker-inserted veneers call sites: "
              << VeneerCallers << "\n");
+  (void)VeneerCallers;
 }
 
 } // namespace bolt