Speedup the V4 code generation.

Speedup the V4 code generation.

Skip discarded code and use QVarLengthArray instead of
QByteArray to store the bytecode.

Change-Id: I4c574e7a817595bc8942ed9a927e79339a2d7b40
Reviewed-on: http://codereview.qt.nokia.com/3760
Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>