Priority list for OptimizeInstructions reviews

[MaxGraey]

@kripken suggested creating a list of PRs here, sorted in order of priority (the most important first):

• [OptimizeInstructions] Canonicalize relational ops with near zero on rhs #4272
• [OptimizeInstructions] Combine extend to i64 and 32-bit load operations #4307
• [OptimizeInstructions] Canonicalizations for relational ops near signed & unsigned min/max #4282
• [OptimizeInstructions] Transform 64-bit low word masking to wrap + extend unary ops #4145
• [Costs] Increase costs for rem by const operations #4291
• [OptimizeInstruction] Sync minimum cost for conditionalizeExpensiveOnBitwise with selectify #4266
• [Costs] Differentiate unary costs #4249

I think that's enough for now. I then plan to review and update older PRs

[kripken]

Thanks, I think this will help organize things!