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[VPlan, SLP] Add simple SLP analysis on top of VPlan.

Description

[VPlan, SLP] Add simple SLP analysis on top of VPlan.

This patch adds an initial implementation of the look-ahead SLP tree
construction described in 'Look-Ahead SLP: Auto-vectorization in the Presence
of Commutative Operations, CGO 2018 by Vasileios Porpodas, Rodrigo C. O. Rocha,
Luís F. W. Góes'.

It returns an SLP tree represented as VPInstructions, with combined
instructions represented as a single, wider VPInstruction.

This initial version does not support instructions with multiple
different users (either inside or outside the SLP tree) or
non-instruction operands; it won't generate any shuffles or
insertelement instructions.

It also just adds the analysis that builds an SLP tree rooted in a set
of stores. It does not include any cost modeling or memory legality
checks. The plan is to integrate it with VPlan based cost modeling, once
available and to only apply it to operations that can be widened.

A follow-up patch will add a support for replacing instructions in a
VPlan with their SLP counter parts.

Reviewers: Ayal, mssimpso, rengolin, mkuper, hfinkel, hsaito, dcaballe, vporpo, RKSimon, ABataev

Reviewed By: rengolin

Differential Revision: https://reviews.llvm.org/D4949