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Double check that passes correctly set their Modified status

Authored by serge-sans-paille on May 28 2020, 3:40 AM.

Description

Double check that passes correctly set their Modified status

The approach is simple: if a pass reports that it's not modifying a
Function/Module, compute a loose hash of that Function/Module and compare it
with the original one. If we report no change but there's a hash change, then we
have an error.

This approach misses a lot of change but it's not super intrusive and can
detect most of the simple mistakes.

Differential Revision: https://reviews.llvm.org/D80916