BasicAA: Only query twice if the result of the more general query was MayAlias

This is a compile time optimization. Before the patch we would do two traversals
on each call to aliasGEP - one with a set size parameter one with UnknownSize.
We can do better by first checking the result of the alias query with
UnknownSize.
Only if this one returns MayAlias do we query a second time using size and type.

This recovers an about 7% compile time regression on spec/ammp.

radar://12349960

llvm-svn: 178045