glsl: Fix chained assignments of vector channels.

glsl: Fix chained assignments of vector channels.

Simple shaders such as:

    void splat(vec2 v, float f) {
        v[0] = v[1] = f;
    }

failed to compile with the following error:
error: value of type vec2 cannot be assigned to variable of type float

First, we would process v[1] = f, and transform:
LHS: (expression float vector_extract (var_ref v) (constant int (1)))
RHS: (var_ref f)
into:
LHS: (var_ref v)
RHS: (expression vec2 vector_insert (var_ref v) (constant int (1))
                 (var_ref f))

Note that the LHS type is now vec2, not a float.  This is surprising,
but not the real problem.

After emitting assignments, this ultimately becomes:
(declare (temporary) vec2 assignment_tmp)
(assign (xy)
  (var_ref assignment_tmp)
  (expression vec2 vector_insert (var_ref v) (constant int (1))
              (var_ref f)))
  (assign (xy) (var_ref v) (var_ref assignment_tmp))

We would then return (var_ref assignment_tmp) as the rvalue, which has
the wrong type---it should be float, but is instead a vec2.

To fix this, we simply return (vector_extract (var_ref assignment_temp)
<the appropriate channel>) to pull out the desired float value.

Fixes Piglit's chained-assignment-with-vector-constant-index.vert and
chained-assignment-with-vector-dynamic-index.vert tests.

Cc: mesa-stable@lists.freedesktop.org
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=74026
Reported-by: Dan Ginsburg <dang@valvesoftware.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>