ra: Fix register spilling.

ra: Fix register spilling.

Commit 551c991606e543c3a264a762026f11348b37947e tried to avoid spilling
registers that were trivially colorable.  But since we do optimistic
coloring, the top of the stack also contains nodes that are not trivially
colorable, so we need to consider them for spilling (since they are some
of our best candidates).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=58384
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=63674
NOTE: This is a candidate for the 9.1 branch.