Implement full debug-info support in new x64 backend

[cfallin]

We need to add support for generating ValueLabelRanges to the machinst framework and (at least) the x64 backend before we can remove the x86 backend (#1936, #2079).

This requires tracking the correspondence from CLIF Values to VCode Regs (pre- and post-regalloc) and SpillSlots. We will need to either:

• extend the returned info from regalloc.rs somewhat in order to provde the pre-to-post-regalloc mapping, or
• stash this pre-to-post-regalloc mapping on our side inside the map_regs() callback, possibly only for Regs that correspond to ValueLabels

[bjorn3]

I think this is done.

[cfallin]

Indeed, completed recently, forgot to close!