glsl: Don't mark assignment temporaries as read-only

glsl: Don't mark assignment temporaries as read-only

The various l-value errors this was designed to catch are now caught
by other means.  Marking the temporaries as read-only now just
prevents sensible error messages from being generated.  It's

0:0(0): error: function parameter 'out p' references the read-only variable '_post_incdec_tmp'

versus

0:13(5): error: function parameter 'out p' references a post-decrement operation

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>