glsl: Do not kill dead assignments to buffer variables or SSBO declarations.

glsl: Do not kill dead assignments to buffer variables or SSBO declarations.

If we kill dead assignments we lose the buffer writes.

Also, we never kill UBO declarations even if they are never referenced
by the shader, they are always considered active. Although the spec
does not seem say this specifically for SSBOs, it is probably implied
since SSBOs are pretty much the same as UBOs, only that you can write
to them.

v2:
- Fix the comment (Jordan)

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>