glsl2: Write vector constructor constants in a single assignment

Make two passes over the constructor parameters.  Write all of the
constants in a single write, then write the non-constants one at a
time.  This causes the fragment shader

varying float g;
void main()
{
	gl_FragColor = vec4(0.0, g, 0.0, 1.0);
}

to generate

(function main
  (signature void (parameters )
    (
      (declare (temporary ) vec4 vec_ctor@0x8580058)
      (assign (constant bool (1)) (xzw) (var_ref vec_ctor@0x8580058)  (constant vec4 (0.000000 0.000000 0.000000 1.000000)) )
      (assign (constant bool (1)) (y) (var_ref vec_ctor@0x8580058)  (swiz xxxx (var_ref g@0x8580218) ))
      (assign (constant bool (1)) (xyzw) (var_ref gl_FragColor@0x84d32a0)  (var_ref vec_ctor@0x8580058) )
    ))
)

instead of

(function main
  (signature void (parameters )
    (
      (declare (temporary ) vec4 vec_ctor@0x8580058)
      (assign (constant bool (1)) (x) (var_ref vec_ctor@0x8580058)  (constant vec4 (0.000000 0.000000 0.000000 1.000000)) )
      (assign (constant bool (1)) (y) (var_ref vec_ctor@0x8580058)  (swiz xxxx (var_ref g@0x8580218) ))
      (assign (constant bool (1)) (z) (var_ref vec_ctor@0x8580058)  (constant vec4 (0.000000 0.000000 0.000000 1.000000)) )
      (assign (constant bool (1)) (w) (var_ref vec_ctor@0x8580058)  (constant vec4 (0.000000 0.000000 0.000000 1.000000)) )
      (assign (constant bool (1)) (xyzw) (var_ref gl_FragColor@0x84d32a0)  (var_ref vec_ctor@0x8580058) )
    ))
)

A similar optimization could be done for matrix constructors, but it
is a little more complicate there.

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