Bitwise operators should overflow on 32bits

@ncannasse [said](https://github.com/HaxeFoundation/haxe/issues/7533#issuecomment-432648995):
> all bitwise operations (<< >> & | ^ ) should properly overflow on 32 bits.
> we should specify this in the doc.

If this is our spec, then we need additional tests in our test suite and the php target should comply (which [will make bitwise operators on php ~13% slower](https://github.com/HaxeFoundation/haxe/issues/7533#issuecomment-479413138)).

If so, this is better to be done in 4.0 than in 4.1