Currently bb::context is intialized (at -1) when the basic block is allocated. But this field: 1) is only used when using the sparse tools; 2) when used, it's only quite late in the process; 3) this early initialization prevents us to reuse the space of this field for another purpose, earlier in the process. Change this by initializing this field much later, by the sparse tool itself, just before needing it. The real motivation being, of course, to be able to reuse the space for some other upcoming field. Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>