intel/compiler: Fix assertions in brw_alu3

v2: Fix assertion for src1 (Ian Romanick)

Fixes: 3b967e17 (intel/compiler: Avoid false positive assertions)
Signed-off-by: Sagar Ghuge <sagar.ghuge@intel.com>
Suggested-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
(cherry picked from commit 30167563)