i965/vs: Try again when we've successfully spilled a reg.

Before, we'd spill one reg, then continue on without actually register
allocating, then assertion fail when we tried to use a vgrf number as a
register number.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit d4bcc6591812ebe72a363cf98371de5e5016f481)

This should have been picked when 9237f0e was picked.

Bugzill: https://bugs.freedesktop.org/show_bug.cgi?id=59700
3 files changed