Subzero: Merge SpillVariable functionality directly into Variable.

Subzero: Merge SpillVariable functionality directly into Variable.

Specifically, this is the ability to link variable B to variable A, such that if neither A nor B get a register assignment, they share a stack slot.

This CL just refactors and keeps basically the same functionality, in preparation for new work on variable splitting.

BUG= none
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/2107073002 .