glsl: Reassociate multiplication of mat*mat*vec.

glsl: Reassociate multiplication of mat*mat*vec.

The typical case of mat4*mat4*vec4 is 80 scalar multiplications, but
mat4*(mat4*vec4) is only 32.

On HSW (with vec4 vertex shaders):
instructions in affected programs:     4420 -> 3194 (-27.74%)

On BDW (with scalar vertex shaders):
instructions in affected programs:     12756 -> 6726 (-47.27%)

Implementing a general matrix chain ordering is harder (or at least
tedious) because of having to walk the GLSL IR to create a list of
multiplicands. I'm guessing that this patch handles 90+% of cases, but
of course to tell definitively you'd have to implement the general
thing.

Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>