freedreno/a3xx/compiler: avoid negative register ids

freedreno/a3xx/compiler: avoid negative register ids

In some cases, we need a register to be assigned up to three components
before the base.  Since we can't have negative register #'s, just shift
everything up.  May increase register usage for trivial shaders, but I
don't think we are shader limited in those cases.  A proper solution is
going to require a better register assignment algorithm (which is on the
TODO list), this is just a hack to get us by until then.

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