[IVDescriptor] Find original 'Previous' for first-order recurrences.

This patch extends first-order recurrence handling to support cases
where we already sunk an instruction for a different recurrence, but
LastPrev comes before Previous.

To handle those cases correctly, we need to find the earliest entry for
the sink-after chain, because this is references the Previous from the
original recurrence. This is needed to ensure we use the correct
instruction as sink point.

Depends on D118558.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D118642