Disable arm-gepopt by default

[Michael Zolotukhin]

Hi,

Recently I’ve found that LLVM regressed on a TwoFish test (from geekbench) when comparing relatively new compiler and a year-old one. The performance regression is pretty big - about 20%, so I decided to investigate it further and understand what change exposed that. Triaging led me to r222328 and r222331, which introduced a new ARM64-specific pass for hoisting constant offsets from addresses. It claimed some gains on SPECS, but when I turned it off, I saw no regressions there (I checked SPEC2006/ref-input set, built with PGO and LTO).

Moreover, when the patch was committed, it effectively disabled two existing regression tests, which is not good I think.

Is it ok to disable this pass by default and enable the tests back? We can turn it back on when we are confident that all tests pass, and there are no new regressions. Any objections?

Thanks,
Michael

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