i965: Add fs_visitor::run_vs() to generate scalar vertex shader code

i965: Add fs_visitor::run_vs() to generate scalar vertex shader code

This patch uses the previous refactoring to add a new run_vs() method
that generates vertex shader code using the scalar visitor and
optimizer.

Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>