Differential analysis tool for optimizations

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Migrated from riscv-collab/v8#291:

This idea is based on this poster from this year's LLVM Developers' Meeting: https://www.lowrisc.org/blog/2020/10/how-we-used-differential-testing-to-rapidly-find-and-fix-missed-optimisation-opportunities-in-llvms-risc-v-backend/llvm-dev-meeting-oct-2020-poster.pdf

We could implement something similar to help identify optimizations that we are missing in v8 that already exist in gcc and LLVM. This could be implemented by compiling C code, generated by a fuzzer, into WASM, then running it through v8 and dumping the generated assembly code and comparing it to the assembly code which gets generated by LLVM/gcc for the same C code.

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This repo has the code used for the above mentioned poster. I found that it is easy to modify it to work with a different compiler. Of course, our situation is a bit different, since V8 is not just a compiler, but we can still dump generated assembly code, so we should be able to use this tool.