glsl: don't dead code remove SSO varyings marked as active

glsl: don't dead code remove SSO varyings marked as active

GL_ARB_separate_shader_objects allow matching by name variable or block
interface. Input varyings can't be removed because it is will impact the
location assignment.

This fixes the bug 79783 and likely any application that uses
GL_ARB_separate_shader_objects extension.

V2 (by Timothy Arceri):
* simplify now that builtins are not set as always active

Signed-off-by: Gregory Hainaut <gregory.hainaut@gmail.com>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
https://bugs.freedesktop.org/show_bug.cgi?id=79783