[BasicAA] Remove checks for GEP decomposition limit reached

The GEP aliasing code currently checks for the GEP decomposition
limit being reached (i.e., we did not reach the "final" underlying
object). As far as I can see, these checks are not necessary. It is
perfectly fine to work with a GEP whose base can still be further
decomposed.

Looking back through the commit history, these checks were originally
introduced in 1a444489. However, I
believe that the problem this was intended to address was later
properly fixed with 1726fc69, and
the checks are no longer necessary since then (and were not the
right fix in the first place).

Differential Revision: https://reviews.llvm.org/D91010