nir/copy_prop: Respect the source's number of components

nir/copy_prop: Respect the source's number of components

In the near future we are going to require that the num_components in a
src dereference match the num_components of the SSA value being
dereferenced.  To do that, we need copy_prop to not remove our MOVs from
a larger SSA value into an instruction that uses fewer channels.

Because we suddenly have to know how many components each source has,
this makes the pass a bit more complicated.  Fortunately, copy
propagation is the only pass that cares about the number of components
are read by any given source so it's fairly contained.

Shader-db results on Sky Lake:

   total instructions in shared programs: 13318947 -> 13320265 (0.01%)
   instructions in affected programs: 260633 -> 261951 (0.51%)
   helped: 324
   HURT: 1027

Looking through the hurt programs, about a dozen are hurt by 3
instructions and the rest are all hurt by 2 instructions.  From a
spot-check of the shaders, the story is always the same:  They get a
vec4 from somewhere (frequently an input) and use the first two or three
components as a texture coordinate.  Because of the vector component
mismatch, we have a mov or, more likely, a vecN sitting between the
texture instruction and the input.  This means that the back-end inserts
a bunch of MOVs and split_virtual_grfs() goes to town.  Because the
texture coordinate is also used by some other calculation, register
coalesce can't combine them back together and we end up with an extra 2
MOV instructions in our shader.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>