glsl: don't let an 'if' then-branch kill copy propagation (elements) for else-branch

glsl: don't let an 'if' then-branch kill copy propagation (elements) for else-branch

When handling 'if' in copy propagation elements, if a certain variable
was killed when processing the first branch of the 'if', then the
second would get any propagation from previous nodes.

    x = y;
    if (...) {
        z = x;  // This would turn into z = y.
        x = 22; // x gets killed.
    } else {
        w = x;  // This would NOT turn into w = y.
    }

With the change, we let copy propagation happen independently in the
two branches and only then apply the killed values for the subsequent
code.

One example in shader-db part of shaders/unity/8.shader_test:

    (assign  (xyz) (var_ref col_1)  (var_ref tmpvar_8) )
    (if (expression bool < (swiz y (var_ref xlv_TEXCOORD0) )(constant float (0.000000)) ) (
      (assign  (xyz) (var_ref col_1)  (expression vec3 + (var_ref tmpvar_8) ... ) ... )
    )
    (
      (assign  (xyz) (var_ref col_1)  (expression vec3 lrp (var_ref col_1) ... ) ... )
    ))

The variable col_1 was replaced by tmpvar_8 in the then-part but not
in the else-part.

NIR deals well with copy propagation, so it already covered for the
missing ones that this patch fixes.

Reviewed-by: Eric Anholt <eric@anholt.net>