vc4: Take advantage of the 8888 pack function in pack_unorm_4x8.
vc4: Take advantage of the 8888 pack function in pack_unorm_4x8.

One instruction instead of four, and it turns out you do this a lot for
the Over operator.

total uniforms in shared programs: 32168 -> 32087 (-0.25%)
uniforms in affected programs:     318 -> 237 (-25.47%)
total instructions in shared programs: 89830 -> 89472 (-0.40%)
instructions in affected programs:     6434 -> 6076 (-5.56%)

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler