Do not set pre-decided consecutive registers as busy (again)

[kunalspathak]

In #84824, I started marking the previous register assigned during copyReg as live. However, the previous register could be part of the consecutive registers that we are about to assign and marking it "live at this location" will make it unavailable to be assigned to the relevant subsequent RefPosition. Start tracking the consecutive refpositions in use and do not mark such refpositions as live. Alternatively, I could delay this decision until the assignment of the particular Refposition, however, at that point we want to make sure that we are not randomly unmarking a register that was marked as "live" and hide any other unseen problem.

Also fixed a AV that was hitting for JitDump because recently we also started doing copyReg for upperVectorRestore and when we print the interval name, we hit AV because interval->relatedInterval is set to nullptr during register assignment.

Fixes: #85426

[ghost]

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.

Issue Details

---

null

Author:	kunalspathak
Assignees:	kunalspathak
Labels:	area-CodeGen-coreclr
Milestone:	-

[kunalspathak]

/azp run runtime-coreclr jitstress, runtime-coreclr libraries-jitstress, runtime-coreclr jitstress2-jitstressregs,
runtime-coreclr libraries-jitstress2-jitstressregs, runtime-coreclr jitstressregs, runtime-coreclr libraries-jitstressregs

[azure-pipelines]

Azure Pipelines successfully started running 6 pipeline(s).

[kunalspathak]

@dotnet/jit-contrib