[DA] Fix function `findBoundsXX` (e.g., `findBoundsLT`)

This is extracted from https://github.com/llvm/llvm-project/pull/160924 and little bit of a placeholder (more details will follow), but this is an attempt to document all know DA issues:

The complaint about function `findBoundsXX` was the following:

> The equations in the comment above each function (following "Wolfe gives the equations") don't appear to consider overflow

I will add this to the meta issue https://github.com/llvm/llvm-project/issues/161805 that lists all DA issues.

Start at the findBoundsXX functions, such as findBoundsLT, and read the equations in the comments following “Wolfe gives the equations.” Review the linked pull request and the DA meta issue for context. Done requires resolving the documented concern about overflow in those equations, but the issue says that more details will follow.

Written by the indexing model from the issue text.