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[AArch64][GlobalISel] Create a new minimal combiner pass just for -O0.

Authored by aemerson on May 6 2021, 5:14 PM.

Description

[AArch64][GlobalISel] Create a new minimal combiner pass just for -O0.

We never bothered to have a separate set of combines for -O0 in the prelegalizer
before. This results in some minor performance hits for a mode where performance
isn't a concern (although not regressing code size significantly is still preferable).

This also removes the CSE option since we don't need it for -O0.

Through experiments, I've arrived at a set of combines that gets the most code
size improvement at -O0, while reducing the amount of time spent in the combiner
by around 35% give or take.

Differential Revision: https://reviews.llvm.org/D102038