[SCEV] Generalize the SCEV algorithm for creating expressions for PHI nodes

Summary:
When forming expressions for phi nodes having an incoming value from
outside the loop A and a value coming from the previous iteration B
we were forming an AddRec if:
  - B was an AddRec
  - the value A was equal to the value for B at iteration -1 (or equal
    to the value of B shifted by one iteration, at iteration 0)

In this case, we were computing the expression to be the expression of
B, shifted by one iteration.

This changes generalizes the logic above by removing the restriction that
B needs to be an AddRec. For this we introduce two expression rewriters
that allow us to
  - shift an expression by one iteration
  - get the value of an expression at iteration 0

This allows us to get SCEV expressions for PHI nodes when these expressions
are not AddRecExprs.

Reviewers: sanjoy

Subscribers: llvm-commits, sanjoy

Differential Revision: http://reviews.llvm.org/D14175

llvm-svn: 251700