i965/vs: Implement register spilling.

i965/vs: Implement register spilling.

To validate this code, I ran piglit -t vs quick.tests with the "go spill
everything" debugging code enabled.  There was only one regression:
glsl-vs-unroll-explosion simply ran out of registers.  This should be
fine in the real world, since no one actually spills every single
register.

NOTE: This is a candidate for the 9.0 branch. Even if it proves to have
bugs, it's likely better than simply failing to compile.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit 9237f0ea8d176fb5dcd41868dcc723fe34f6b1f3)