[AAPointerInfo] track multiple constant offsets for each use

An expression of the form `gep(base, select(pred, const1, const2))` can result
in a set of offsets instead of just one. PointerInfo can now track these sets
instead of conservatively modeling them as Unknown. In general, AAPointerInfo
now uses AAPotentialConstantValues to examine the operands of the GEP.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D138646