glsl/standalone: Optimize dead variable declarations

glsl/standalone: Optimize dead variable declarations

We didn't bother with this in the regular compiler because it doesn't
change the generated code.  In the stand-alone compiler, this can
clutter the output with useless variables.  It's especially bad after
functions are inlined but the foo_retval declarations remain.

v2: Use set_foreach.  Suggested by Tapani.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>