glsl: Avoid making a temporary for lower_mat_op_to_vec if not needed.

glsl: Avoid making a temporary for lower_mat_op_to_vec if not needed.

Our copy propagation tends to be bad at handling the later array
accesses of the matrix argument we moved to a temporary.  Generally we
don't need to move it to a temporary, though, so this avoids needing
more copy propagation complexity.

Reduces instruction count of some Unigine Tropics and Sanctuary
fragment shaders that do operations on uniform matrix arrays by 5.9%
on gen6.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>