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Add the llvm-canon tool. Description from the [original PR](https://reviews.llvm.org/D66029#change-wZv3yOpDdxIu): > Added a new llvm-canon tool which aims to transform LLVM Modules into a canonical form by reordering and renaming instructions while preserving the same semantics. This tool makes it easier to spot semantic differences while diffing two modules which have undergone different transformation passes. The current version of this tool can: - Reorder instructions within a function. - Rename instructions based on the operands. - Sort commutative operands. This code was originally written by @michalpaszkowski and [submitted to mainline LLVM](14d358537f). However, it was quickly [reverted](335de55fa3) to do BuildBot errors. Michal presented his version of the tool in [LLVM-Canon: Shooting for Clear Diffs](https://www.youtube.com/watch?v=c9WMijSOEUg). @AidanGoldfarb and I ported the code to the new pass manager, added more tests, and fixed some bugs related to PHI nodes that may have been the root cause of the BuildBot errors that caused the patch to be reverted. Additionally, we rewrote the implementation of instruction reordering to fix cases where the original algorithm would break use-def chains. Note that this is @AidanGoldfarb and I's first time submitting to LLVM. Please liberally critique the PR! CC @plotfi for initial review. --------- Co-authored-by: Aidan <aidan.goldfarb@mail.mcgill.ca>