Fix #14540 FP shiftNegative after sign check (#8269)

Author: chrchr-github

lib/valueflow.cpp

@@ -3682,7 +3682,7 @@ static void valueFlowSymbolicOperators(const SymbolDatabase& symboldatabase, con
                     continue;
                 if (vartok->exprId() == 0)
                     continue;
-                if (Token::Match(tok, "<<|>>|/") && !astIsLHS(vartok))
+                if (Token::Match(tok, "<<|>>|/|-") && !astIsLHS(vartok))
                     continue;
                 if (Token::Match(tok, "<<|>>|^|+|-|%or%") && constant->intvalue != 0)
                     continue;

test/testvalueflow.cpp

@@ -8910,6 +8910,18 @@ class TestValueFlow : public TestFixture {
                "    return x;\n"
                "}\n";
         ASSERT_EQUALS(false, testValueOfXKnown(code, 3U, "a", 0));
+
+        code = "void f(int n) {\n"
+               "    int x = 0 - n;\n"
+               "    return x;\n"
+               "}\n";
+        ASSERT_EQUALS(false, testValueOfX(code, 3U, "n", ValueFlow::Value::ValueType::SYMBOLIC));
+
+        code = "void f(int n) {\n"
+               "    int x = n - 0;\n"
+               "    return x;\n"
+               "}\n";
+        ASSERT_EQUALS(true, testValueOfX(code, 3U, "n", ValueFlow::Value::ValueType::SYMBOLIC));
     }
 
     void valueFlowSymbolicStrlen()