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Temporairly revert "[SimplifyCFG][LoopRotate] SimplifyCFG: disable common…

Authored by lebedev.ri on Aug 21 2020, 2:24 PM.

Description

Temporairly revert "[SimplifyCFG][LoopRotate] SimplifyCFG: disable common instruction hoisting by default, enable late in pipeline"

As disscussed in post-commit review starting with
https://reviews.llvm.org/D84108#2227365
while this appears to be mostly a win overall, especially code-size-wise,
this appears to shake certain code pattens in a way that is extremely
unfavorable for performance (+30% runtime regression)
on certain CPU's (i personally can't reproduce).

So until the behaviour is better understood, and a path forward is mapped,
let's back this out for now.

This reverts commit 1d51dc38d89bd33fb8874e242ab87b265b4dec1c.