[BasicAA] Avoid duplicate query for GEPs with identical offsets (NFCI)

For two GEPs with identical offsets, we currently first perform
a base address query without size information, and then if it is
MayAlias, perform another with size information. This is pointless,
as the latter query should produce strictly better results.

This was not quite true historically due to the way that NoAlias
assumptions were handled, but that issue has since been resolved.