glsl2: Make ir_algebraic reassociate add/mul operands for constant folding.

glsl2: Make ir_algebraic reassociate add/mul operands for constant folding.

It's rather easy to produce two constant multiplies separated by other
multiplies while writing a BRDF shader, and non-obvious enough in the
resulting codegen that I didn't catch it in my demo code until just
recently.  Cuts 3 965 instructions from my demo (<1%), and 20 from
glsl-fs-raytrace (1.3%).