Fix false positives with condition with || and &&

The value of something in the middle of a condition with mixed || and &&
gives no information on which branch will be taken.
For instance with:
```
int f(int a, int b, bool x) {\n"
  if (a == 1 && (!(b == 2 && x))) {
  } else {
    if (x) {
    }
  }

  return 0;
}
```
We can enter the if part whether x is true or false, and similarly,
enter the else part whether x is true or false. Same thing with the
value of b.

This fixes the following false positive with above code:
```
:4:13: style: Condition 'x' is always true [knownConditionTrueFalse]
        if (x) {
            ^
:2:33: note: Assuming that condition 'x' is not redundant
    if (a == 6 && (!(b == 21 && x))) {
                                ^

```

lib/valueflow.cpp

@@ -4424,6 +4424,31 @@ struct ValueFlowConditionHandler {
                     }
                 }
 
+                {
+                    const Token *tok2 = tok;
+                    std::string op;
+                    bool mixedOperators = false;
+                    while (tok2->astParent()) {
+                        const Token *parent = tok2->astParent();
+                        if (Token::Match(parent, "%oror%|&&")) {
+                            if (op.empty()) {
+                                op = parent->str() == "&&" ? "&&" : "||";
+                            } else if (op != parent->str()) {
+                                mixedOperators = true;
+                                break;
+                            }
+                        }
+                        if (parent->str()=="!") {
+                            op = (op == "&&" ? "||" : "&&");
+                        }
+                        tok2 = parent;
+                    }
+
+                    if (mixedOperators) {
+                        continue;
+                    }
+                }
+
                 if (top && Token::Match(top->previous(), "if|while (") && !top->previous()->isExpandedMacro()) {
                     // does condition reassign variable?
                     if (tok != top->astOperand2() && Token::Match(top->astOperand2(), "%oror%|&&") &&

test/testvalueflow.cpp

@@ -138,6 +138,8 @@ private:
         TEST_CASE(valueFlowCrash);
         TEST_CASE(valueFlowHang);
         TEST_CASE(valueFlowCrashConstructorInitialization);
+
+        TEST_CASE(valueFlowUnknownMixedOperators);
     }
 
     static bool isNotTokValue(const ValueFlow::Value &val) {
@@ -4804,6 +4806,20 @@ private:
                "}";
         valueOfTok(code, "path");
     }
+
+    void valueFlowUnknownMixedOperators() {
+        const char *code= "int f(int a, int b, bool x) {\n"
+                          "  if (a == 1 && (!(b == 2 && x))) {\n"
+                          "  } else {\n"
+                          "    if (x) {\n"
+                          "    }\n"
+                          "  }\n"
+                          "\n"
+                          "  return 0;\n"
+                          "}" ;
+
+        ASSERT_EQUALS(false, testValueOfXKnown(code, 4U, 1));
+    }
 };
 
 REGISTER_TEST(TestValueFlow)