glsl/lower_output_reads: set invariant and precise flags on temporaries

fixes a couple of deqp tests (on nvc0 and potential other drivers):
dEQP-GLES3.functional.shaders.invariance.highp.common_subexpression_1
dEQP-GLES3.functional.shaders.invariance.highp.common_subexpression_2
dEQP-GLES3.functional.shaders.invariance.highp.common_subexpression_3
dEQP-GLES3.functional.shaders.invariance.mediump.common_subexpression_1
dEQP-GLES3.functional.shaders.invariance.mediump.common_subexpression_2
dEQP-GLES3.functional.shaders.invariance.mediump.common_subexpression_3
dEQP-GLES3.functional.shaders.invariance.lowp.common_subexpression_1
dEQP-GLES3.functional.shaders.invariance.lowp.common_subexpression_2
dEQP-GLES3.functional.shaders.invariance.lowp.common_subexpression_3

CC: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
(cherry picked from commit 987744be)