glsl: Rename lower_mat_op_to_vec operands/results to be less hungarian.

glsl: Rename lower_mat_op_to_vec operands/results to be less hungarian.

This awkward typing was to avoid shadowing the function argument (the
matrix) with the temporary deref (the column) before the
get_column()/get_element()s were moved into the expression/assignment
constructors.  They're about to become not-variables, so the current
names had to go.  This change is almost mechanical (other than
column_expr), so it should make the next diff clearer.

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