[Relax] Exclude R.null_value()-bound vars from KillAfterLastUse

[cchung100m]

Hi Committers,

This PR addresses issue #20200. Any suggestions would be appreciated if you are available.

Root Cause

KillAfterLastUse decides whether a variable is a legal R.vm.kill_object target via stored_in_vm_register, which only excludes constants and FuncType/ShapeType/PrimType vars. A variable bound to R.null_value() has type AnyType - not in that exclusion list - so the pass wrongly treats it as killable. But both CodeGenVM and CodeGenVMTIR special-case null_value to bypass real VM register/anylist-slot allocation entirely. In CodeGenVMTIR, this mismatch hits an ICHECK requiring the kill target to resolve to an anylist_getitem() call, causing a hard crash (CodeGenVM's bytecode backend happens to swallow it silently via a sentinel "void register," which is why only exec_mode="compiled" crashes).

Solution

Track variables bound to relax.null_value in CollectLastUsage (mirroring the existing constant_tensors_ pattern) and add them to the stored_in_vm_register exclusion, so the pass never emits a kill for them - closing the gap at its sources rather than patching either codegen.