[Pipeliner] Check for affine expression in isLoopCarriedOrder

The pipeliner must add a loop carried dependence between two memory
operations if the base register is not an affine (linear) exression.
The current implementation doesn't check how the base register is
defined, which allows non-affine expressions, and then the pipeliner
does not add a loop carried dependence when one is needed.

This patch adds code to isLoopCarriedOrder that checks if the base
register of the memory operations is defined by a phi, and the loop
definition for the phi is a constant increment value.  This is a very
simple check for a linear expression.

Patch by Brendon Cahoon.

llvm-svn: 328550