glsl: Extend the array splitting optimization pass to matrices.

glsl: Extend the array splitting optimization pass to matrices.

This should fit in well with our lower_mat_op_to_vec code: now, in
addition to having expressions on each column of a matrix, we also
split the columns to separate variables so they can be tracked
individually by the copy propagation, dead code, and other passes.

This optimizes out some more code generation in unigine and gstreamer
shaders.

Total instructions: 269342 -> 269270
14/2148 programs affected (0.7%)
2226 -> 2154 instructions in affected programs (3.2% reduction)