[basicaa] Rewrite isGEPBaseAtNegativeOffset in terms of index difference [mostly NFC]

This is almost purely NFC, it just fits more obviously in the flow of the code now that we've standardized on the index different approach.  The non-NFC bit is that because of canceling the VariableOffsets in the subtract, we can now handle the case where both sides involve a common variable offset.  This isn't an "interesting" improvement; it just happens to fall out of the natural code structure.

One subtle point - the placement of this above the BaseAlias check is important in the original code as this can return NoAlias even when we can't find a relation between the bases otherwise.

Also added some enhancement TODOs noticed while understanding the existing code.

Note: This is slightly different than the LGTMed version.  I fixed the "inbounds" issue Nikita noticed with the original code in e6e5ef40 and rebased this to include the same fix.

Differential Revision: https://reviews.llvm.org/D97520