[LAA] Keep track of ptr SCEV related to MemAccessInfo (WIP).

[fhahn]

Summary

This patch extends MemAccessInfo and PointerInfo to track the associated
SCEV for the pointer. The SCEV expressions is then used instead of
accessing ScalarEvolution on demand.

This has multiple benefits:

1. We only need to replace symbolic strides once, up front.
2. Allows for more flexibility to have accesses with different SCEV expressions for their pointers.For example, it allows adding 2 MemAccessInfo for a pointer fed by a select.

This is still a bit rough around the edges. In particular the printing
is still missing the SCEV expression.