i965/fs: Invalidate live intervals in passes that remove an instruction.

Since live intervals are based on ip, removing an instruction trashes
the intervals unless we were to go do some surgery.  These happen to
usually remove a use of a grf, so it's time to recalculate, anyway.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
NOTE: This is a candidate for the 8.0 release branch.

src/mesa/drivers/dri/i965/brw_fs.cpp

@@ -1844,6 +1844,9 @@ fs_visitor::compute_to_mrf()
       }
    }
 
+   if (progress)
+      live_intervals_valid = false;
+
    return progress;
 }
 
@@ -1920,6 +1923,9 @@ fs_visitor::remove_duplicate_mrf_writes()
       }
    }
 
+   if (progress)
+      live_intervals_valid = false;
+
    return progress;
 }