[FEATURE REQUEST] Optimize integer arithmetic

[vouillon]

We could eliminate some unnecessary conversions between 31- and 32-bits integers.

Binaryen does not optimize this at all at the moment. We could implement a pass in Binaryen and/or perform this optimization in Wasm_of_ocaml.

For instance,

(ref.i31 (i32.add (i31.get_s (ref.i31 (local.get $x))) (local.get $y)))

is equivalent to

(ref.i31 (i32.add (local.get $x) (local.get $y)))

We might be able to do more optimizations in Wasm_of_ocaml since we know better the structure of the code that we generate (keep the index of a for loop in a 32-bit integer, interprocedural optimization, ...)