i965/vs: Allocate register set once at context creation.

i965/vs: Allocate register set once at context creation.

Now that we use a fixed set of register classes, we can set up the
register set and conflict graphs once, at context creation, rather than
on every VS compile.  This is obviously less expensive, and also what
we already do in the FS backend.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>