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[SCEVExpander] Stop hoisting IR when reusing phis

Authored by reames on Aug 17 2021, 9:26 AM.

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[SCEVExpander] Stop hoisting IR when reusing phis

his is a fix for PR43678, and is an alternate patch to D105723.

The basic issue we're running into is that LSR + SCEVExpander are moving the very instruction whose operand we're in the process of expanding. This breaks the subtle and ill-documented invariant which let LSR work. (Full story can be found here: https://reviews.llvm.org/D105723#2878473)

Rather than attempting a fix, this change just removes the optimization entirely. The code is entirely untested, and removing it appears to have no impact I can find. This code was added back in 2014 by 1e12f8563d4b7 with a single test which does not seem to actually test the hoisting logic.

From a philosophical standpoint, it also seems very strange to have the expander implementing optimizations which should live in a dedicated transform pass.

Differential Revision: https://reviews.llvm.org/D106178

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