Questions regarding UInt

Working on passing #4870, I wonder which operations should be performed in "unsigned mode".
For instance let's look at the following:

``` haxe
var x : UInt = 0x80000000;
return x > 0; // unsigned for sure

var y : Int = 0;
return x > y; // signed ? unsigned ?
```

ATM, I'm only using unsigned if the two operands are unsigned or constant int, expect for `%` and `>>` where only the left operand sign is checked.

This would definitely require some extra unit tests to ensure a cross platform behavior.