glsl2: Add a new pass at the IR level to break down matrix ops to vector ops.

This will be used by the Mesa IR and likely most HW backends, as it
allows other optimizations to occur that might not otherwise.

Fixes glsl-vs-mat-sub-1, glsl-vs-mat-div-1.

src/mesa/shader/ir_to_mesa.cpp

@@ -1960,6 +1960,7 @@ _mesa_glsl_compile_shader(GLcontext *ctx, struct gl_shader *shader)
       _mesa_ast_to_hir(shader->ir, state);
 
    /* Lowering */
+   do_mat_op_to_vec(shader->ir);
    do_mod_to_fract(shader->ir);
    do_div_to_mul_rcp(shader->ir);