Enhance "fast unsafe" compilation option

[bushidocodes]

There are some corners of the WebAssembly spec that are strict and explicit where the C and LLVM specifications remain undefined. An example of this is UINT32_MIN % -1, which overflows with undefined semantics in C and LLVM, but which correctly returns 0 in WebAssembly. Achieving strict compliance thus requires special shims beyond LLVM intrinsics. There may be a performance cost to this.

We can enhance our fast unsafe math compiler option to use a "close enough" LLVM intrinsic in place of a totally compliant implementation in certain cases.

[bushidocodes]

Based on https://github.com/sunfishcode/wasm-reference-manual/blob/master/WebAssembly.md#integer-remainder-signed, the wasm spec differs from modulo operations. There is an llvm srem intrinsic, but it seems to track C and have undefined behavior with INT32_MIN and -1.

My logic is just that since division/multiplication can be performed by repeated subtraction/addition, we can add the absolute value of the denominator to be sure that the numerator doesn't overflow, and that results in a numerator between {INT32_MIN, 0}. Since we can't produce a positive number, I don't think that should cause any sign issues.

Here's a REPL with some experiments: https://replit.com/@spmcbride1201/i32rem#main.c

Perhaps we might want to enable to a "quirks mode" where we drop strict compliance for the wasm spec in certain cases? Sort of like how bash takes a flag for strict POSIX shell standards compliance. I suggest we kick that discussion to an issue. It seems like a good idea to try to target strict compliance first and then consciously downshift if there are perf wins for doing so.