[SCEV] Simplify/generalize howFarToZero solving.

Make SolveLinEquationWithOverflow take the start as a SCEV, so we can
solve more cases. With that implemented, get rid of the special case
for powers of two.

The additional functionality probably isn't particularly useful,
but it might help a little for certain cases involving pointer
arithmetic.

Differential Revision: https://reviews.llvm.org/D28884



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@293576 91177308-0d34-0410-b5e6-96231b3b80d8