Fix FP in go/useless-assignment-to-local

Author: owen-mc

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/main.go

@@ -27,7 +27,7 @@ func deadParameter(x int) bool { // we don't want to flag x here
 }
 
 func usedOnlyAfterFalse() bool {
-	s := deadStore() // $ SPURIOUS: Alert
+	s := deadStore()
 	return false && s == 0
 }