glsl: Copy function out to temp if we don't directly ref a variable

glsl: Copy function out to temp if we don't directly ref a variable

Otherwise we can end up with IR that looks like this:

    (
      (declare (temporary ) vec4 f@8)
      (assign  (xyzw) (var_ref f@8)  (var_ref f) )
      (call f16  ((swiz y (var_ref f@8) )))

      (assign  (xyzw) (var_ref f)  (var_ref f@8) )
    ))

When we really need:

      (declare (temporary ) float inout_tmp)
      (assign  (x) (var_ref inout_tmp)  (swiz y (var_ref f) ))
      (call f16  ((var_ref inout_tmp) ))

      (assign  (y) (var_ref f)  (swiz y (swiz xxxx (var_ref inout_tmp) )))
      (declare (temporary ) void void_var)

The GLSL IR function inlining code seemed to produce correct code
even without this but we need the correct IR for GLSL IR -> NIR to
be able to understand whats going on.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>