[bugpoint] Allow the user to specify the path to opt on the commandline.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187739 91177308-0d34-0410-b5e6-96231b3b80d8
Change parent hashing algorithm to be non-recursive and elaborate
greatly on many comments in the code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187742 91177308-0d34-0410-b5e6-96231b3b80d8