[REG 5.6.0->5.6.1] lastIndexOf on a string always returns -1 in Release build

When using the 'lastIndexOf' method of a string in JavaScript, it always returns -1. It only affects release builds. This bug appeard with 5.6.1, 5.6.0 was still ok.

Steps to reproduce:

In 'helloscript' sample, replace the content of the file 'helloscript.js' with:

//! [0]
button.text = "'abcabc'.lastIndexOf('c'): " + 'abcabc'.lastIndexOf('c');
button.styleSheet = 'font-style: italic';
button.show();
//! [0]

The debug build correctly outputs:

'abcabc'.lastIndexOf('c'): 5

The release build outputs:

'abcabc'.lastIndexOf('c'): -1