Fix BasicAA incorrect assumption on GEP

This is fixing pr31761: BasicAA is deducing NoAlias
on the result of the GEP if the base pointer is itself NoAlias.

This is possible only if the NoAlias on the base pointer is
deduced with a non-sized query: this should guarantee that
the pointers are belonging to different memory allocation
and that the GEP can't legally jump from one to another.

Differential Revision: https://reviews.llvm.org/D29216

llvm-svn: 293293