code that I've been using to narrow down a miscompilation, that I believe will
be useful in the future

it has three parts
- build compiler-rt so that we can use UBSan from our LLVM version
- detailed srcloc printing for replaced instructions
- two command line options supporting binary search for a bogus
  transformation
fix debug options to mention souper and to be hidden
Merge pull request #39 from regehr/pass-debug

minor changes to make it easier to debug miscompilations