A LiveRange is identified by 2 integers: the vreg() of its TopLevel,
which is the virtual register (operand) ID; and a relative_id(), which has
no meaning in the program, but is valuable in debugging or tracing
scenarios.

This change ensures that relative_id is unique even in cases of splinter
ranges and their children.

Review URL: https://codereview.chromium.org/1318493005

Cr-Commit-Position: refs/heads/master@{#30665}