Add instructions for use with Rust/Cargo

[saethlin]

@regehr I suspect the equivalent of these instructions will work to get a follow-on superoptimizer working with Rust/Cargo. If anyone tries these out please do let me know how it goes.

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[chadbrewbaker]

Not sure this is how I would go about it. Might be better to do a LLVM IR build like you do for GraalVM/Klee, then whitelist the files you want Souper to optimize? Doing Souper on cold paths doesn't make computational sense.

One feature I would really love is a --souper-verbose-asm that pretty prints transforms it found in the assembler file.

My ideal workflow is compile for profiling, run profiling load to whitelist 99% runtime hot functions, send just those to Souper with a hyperfine style report of the improvement.

Another thing I Souper may be doing is screwing up cache alignments and creating more branch mispredictions?

@ornata - is there an LLVM annotation to keep functions on cache line boundaries, or do thin-BOLT where you re-order small hot functions used together?