glsl: Use constant_initializer instead of constant_value to determine whether to...

glsl: Use constant_initializer instead of constant_value to determine whether to keep an unused uniform

This even matches the comment "uniform initializers are precious, and
could get used by another stage."

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Cc: "10.6 11.0" <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 5bc68f0f2b80b21997435742af74c49eb72891f7)