Add test for FP result when read is unreachable

Author: owen-mc

go/ql/test/query-tests/RedundantCode/DeadStoreOfLocal/CONSISTENCY/UnexpectedFrontendErrors.expected

@@ -1 +1 @@
-| main.go:36:9:36:9 | undefined: unknownFunction |
+| main.go:41:9:41:9 | undefined: unknownFunction |

go/ql/test/query-tests/RedundantCode/DeadStoreOfLocal/main.go

@@ -26,6 +26,11 @@ func deadParameter(x int) bool { // we don't want to flag x here
 	return true
 }
 
+func usedOnlyAfterFalse() bool {
+	s := deadStore() // $ SPURIOUS: Alert
+	return false && s == 0
+}
+
 func test2(x int) (int, int) {
 	y := x >> 5
 	z := x % (1)