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Reland "[SCEVExpander] Add option to preserve LCSSA directly."

Authored by fhahn on Jul 29 2020, 11:35 AM.

Description

Reland "[SCEVExpander] Add option to preserve LCSSA directly."

This reverts the revert commit dc2867576886247cbe351e7c63618c09ab6af808.

It includes a fix for Polly, which uses SCEVExpander on IR that is not
in LCSSA form. Set PreserveLCSSA = false in that case, to ensure we do
not introduce LCSSA phis where there were none before.