Detect more augmentable operators

[laurentlb]

a=3+a => a+=3

[therontarigo]

This is broken for noncommutative operations:
Consider the case of matrix-vector multiplication:

mat3 M;
vec3 v;
v = M*v;

v*=M performs v = v*M, so it is not a correct transformation.

Indeed, it is broken in several of the tests and furthermore the tests have been overwritten with broken results:
343efc4#r141691262