[BOLT] Always call fixBranches in relocation mode.

Summary:
If you attempted to use a function filter on a binary when in relocation mode, the resulting binary would probably crash.  This is because we weren't calling fixBranches on all functions.  This was breaking bughunter.sh

I also strengthened the validation of basic blocks.  The cond branch should always be non-null when there are two successors.

(cherry picked from FBD6261930)

Author: Bill Nell

bolt/BinaryBasicBlock.cpp

@@ -104,7 +104,7 @@ bool BinaryBasicBlock::validateSuccessorInvariants() {
         break;
       case 2:
         Valid =
-          (!CondBranch ||
+          (CondBranch &&
            (TBB == getConditionalSuccessor(true)->getLabel() &&
             ((!UncondBranch && !FBB) ||
              (UncondBranch &&

bolt/Passes/BinaryPasses.cpp

@@ -516,7 +516,7 @@ void FixupBranches::runOnFunctions(
   std::set<uint64_t> &) {
   for (auto &It : BFs) {
     auto &Function = It.second;
-    if (shouldOptimize(Function)) {
+    if (opts::Relocs || shouldOptimize(Function)) {
       Function.fixBranches();
     }
   }