Optimiser should be commutativity/associativity aware

[stevehalliwell]

Presently the optimiser will registerise based on pattern of is the previous instruction a GET_LOCAL take it internal and remove that, if the next previous is a GET_LOCAL, take that too and remove it.

The issue is this catches

var a = 1;
var b = 2;
var c = 0;
c = a + b;

but not

var i = 0;
i += 1;