freedreno/ir3: fix grouping issue w/ reverse swizzles

freedreno/ir3: fix grouping issue w/ reverse swizzles

When we have something like:

   MOV OUT[n], IN[m].wzyx

the existing grouping code was missing a potential conflict.  Due to
input needing to be sequential scalar regs, we have:

 IN:  x <-> y <-> z <-> w

which would be grouped to:

 OUT: w <-> z2 <-> y2 <-> x  (where the 2 denotes a copy/mov)

but that can't actually work.  We need to realize that x and w are
already in the same chain, not just that they aren't both already in
new chain being built.

With this fixed, we probably no longer need the hack from f68f6c0.

Signed-off-by: Rob Clark <robclark@freedesktop.org>