[BasicAA] Avoid calling GetUnderlyingObject, when the result of a previous call can be reused.

Recursive calls to aliasCheck from alias[GEP|Select|PHI] may result in a second call to GetUnderlyingObject for a Value, whose underlying object is already computed. This patch ensures that in this situations, the underlying object is not computed again, and the result of the previous call is resued.

https://reviews.llvm.org/D22305

llvm-svn: 278519