Go: fix FP in go/dead-store-of-local

go/ql/src/RedundantCode/DeadStoreOfLocal.ql

@@ -37,8 +37,11 @@ where
   def.getBasicBlock() instanceof ReachableBasicBlock and
   // exclude assignments with default values or simple expressions
   not isSimple(rhs) and
-  // exclude variables that are not used at all
-  exists(target.getAReference()) and
+  // exclude variables that are not used in reachable code
+  exists(IR::Instruction ref |
+    ref != def and
+    (ref.reads(target) or ref.writes(target, _))
+  ) and
   // exclude variables with indirect references
   not target.mayHaveIndirectReferences() and
   // Report the assigned variable rather than the whole write instruction. A write to an

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()
+	return false && s == 0
+}
+
 func test2(x int) (int, int) {
 	y := x >> 5
 	z := x % (1)