wasm2asm: Finish i64 lowering operations (#1563)

* wasm2asm: Finish i64 lowering operations

This commit finishes out lowering i64 operations to JS with implementations of
division and remainder for JS. The primary change here is to have these compiled
from Rust to wasm and then have them "linked in" via intrinsics. The
`RemoveNonJSOps` pass has been updated to include some of what
`I64ToI32Lowering` was previously doing, basically replacing some instructions
with calls to intrinsics. The intrinsics are now all tracked in one location.

Hopefully the intrinsics don't need to be regenerated too much, but for
posterity the source currently [lives in a gist][gist], although I suspect that
gist won't continue to compile and work as-is for all of time.

[gist]: https://gist.github.com/alexcrichton/e7ea67bcdd17ce4b6254e66f77165690

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