Give std::map<BasicBlock *, BasicBlock *> the short name BBMap, because

Give std::map<BasicBlock *, BasicBlock *> the short name BBMap, because
 it's used 7 different times.
Rename `getBackEdges' to `findAndInstrumentBackEdges', for clarity.
Remove some excess whitespace and commented-out code.
Use shorter forms of CallInst ctors.
Do not make `reopt_threshold' visible to the LLVM program, and do not
 pass it to the call to `reoptimizerInitialize'.  Don't pass the
 GlobalVariable representing it to any of our helper methods.
 `reopt_threshold' is an internal parameter of the reoptimizer, which
 InstLoops does not need to know about.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@7794 91177308-0d34-0410-b5e6-96231b3b80d8