[LV] Support fixed order recurrences.

If the incoming previous value of a fixed-order recurrence is a phi in
the header, go through incoming values from the latch until we find a
non-phi value. Use this as the new Previous, all uses in the header
will be dominated by the original phi, but need to be moved after
the non-phi previous value.

At the moment, fixed-order recurrences are modeled as a chain of
first-order recurrences.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D119661