i965/vec4: Simplify opt_register_coalesce() using the swizzle utils.
i965/vec4: Simplify opt_register_coalesce() using the swizzle utils.

Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler