pan/midgard: Add a dummy source for loads

pan/midgard: Add a dummy source for loads

We want symmetry between loads and stores, so we add a dummy source. So
we get, e.g.

   st_int4 _,    val, arg_1, arg_2
   ld_int4 dest,   _, arg_1, arg_2

Semantically, this dummy source represents the data itself, as if the
load is simply a move. That means it has a swizzle that acts as a
source.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>