Fix S1643 FN: consider simple assignments with variable not on the innermost add expression

[cristian-ambrosini-sonarsource]

Update S1643 to cover simple assignments where the variable is not placed in the innermost add expression. Currently, S1643 only covers cases where the variable is placed at the left side of the innermost add expression:

var s = "";

while (true)
{
    s = s + "a";                     // Noncompliant
    s = s + "a" + "b";           // Noncompliant
    s = s + "a" + "b" + "c";  // Noncompliant 

    s = "a" + "b" + "c" + s;   // FN
    s = "a" + "b" + s;            // FN
    s = "a" + s;                     // FN
}