glsl/opt_cpe: Fix a crash when a kill kills for two reasons.

Fixes glsl-copy-propagation-loop-2 when this optimization pass is
re-enabled.

Reported-by: David Lamparter <equinox@diac24.net>

src/glsl/opt_copy_propagation_elements.cpp

@@ -390,8 +390,10 @@ ir_copy_propagation_elements_visitor::kill(kill_entry *k)
 
       if (entry->lhs == k->var) {
 	 entry->write_mask = entry->write_mask & ~k->write_mask;
-	 if (entry->write_mask == 0)
+	 if (entry->write_mask == 0) {
 	    entry->remove();
+	    continue;
+	 }
       }
       if (entry->rhs == k->var) {
 	 entry->remove();