glsl: Add IR builder support for conditional assignments.

glsl: Add IR builder support for conditional assignments.

This adds two new signatures:

   assign(lhs, rhs, condition, writemask);
   assign(lhs, rhs, condition);

All the other existing APIs still exist.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>