Recommit "[SCEV] Look through single value PHIs." (take 2)

This reverts commit 8fdac7cb.

The issue causing the revert has been fixed a while ago in 60b85209.

Original message:

    Now that SCEVExpander can preserve LCSSA form,
    we do not have to worry about LCSSA form when
    trying to look through PHIs. SCEVExpander will take
    care of inserting LCSSA PHI nodes as required.

    This increases precision of the analysis in some cases.

    Reviewed By: mkazantsev, bmahjour

    Differential Revision: https://reviews.llvm.org/D71539