[DSE] Add support for not aligned begin/end

This is an attempt to improve handling of partial overlaps in case of unaligned begin\end.

Existing implementation just bails out if it encounters such cases. Even when it doesn't I believe existing code checking alignment constraints is not quite correct. It tries to ensure alignment of the "later" start/end offset while should be preserving relative alignment between earlier and later start/end.

The idea behind the change is simple. When start/end is not aligned as we wish instead of bailing out let's adjust it as necessary to get desired alignment.

I'll update with performance results as measured by the test-suite...it's still running...

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D93530