Rename V4IR::InstructionSelection to V4IR::IRDecoder.

Now it can be used by classes that want to walk the IR but do not
generate instructions, without causing confusion.

Change-Id: Idb220a1aee3ba2bbcd43bdeecd28946d567c9f3c
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Introduce PhysicalRegister and StackSlot Temps.

Change-Id: I8d0b6a1e85fd0c42772f294fd13bc6041d41c81e
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>