Fix leaks in Phi nodes in the V4 IR

The phi nodes store a QVector, but as the destructors of the IR nodes aren't
called, the QVector internal data is leaked. This patch re-uses the existing
Stmt::Data mechanism that was introduced to serve exactly that purpose. It
replaces the now unused QVectors there with the Phi::incoming vector and
adjusts usage accordingly.

Change-Id: I22f351a17c1983637b54fa73a93dd40d64fec46c
Reviewed-by: Lars Knoll <lars.knoll@digia.com>