st/mesa: small optimization in swizzle_swizzle()
st/mesa: small optimization in swizzle_swizzle()

Usually, there's no user-specified texture swizzle so we can optimize
the swizzle_swizzle() function and skip the loop/switch.

Reviewed-by: Charmaine Lee <67ce69700661ea856134ddb10419a330e5f037d7@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler