Compress temp usage in the interpreter.

Compress temp usage in the interpreter.

Uses a variation on linear scan register allocation as the algorithm. As
it depends on liveness analysis, keep that data around after codegen is
finished (in IR::Stmt::Data). Added clean-up code for it in the
IR::Function destructor.

Change-Id: If3636648efbafcc1df469a24aaa885e21e6a2f16
Reviewed-by: Lars Knoll <lars.knoll@digia.com>