target-alpha: Use separate TCGv temporaries for the shadow registers

target-alpha: Use separate TCGv temporaries for the shadow registers

This avoids having to manually swap them around when swapping to and
from PALmode.  We simply encode the shadow registers into the translation.

The VMStateDescription version changes, because the meaning of "shadow"
changes in the save file when in PALmode.  It would be possible to fix
this, but I don't think it's worth the effort.

Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Richard Henderson <rth@twiddle.net>