Change the V4 IR so it can be stored in a memory pool.

Change the V4 IR so it can be stored in a memory pool.

That is, remove members with a non-trivial destructor from IR::Stmt
and IR::Expr (that's because we don't destroy the objects
allocated in the pool).

Added the method MemoryPool::reset() so we can clear the pool
without disposing the allocated memory.

Change-Id: I126332be387c016578c086db8b3aa8098b2507f6
Reviewed-on: http://codereview.qt.nokia.com/3758
Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>