JIT: skip stack allocation for unused locals

[AndyAyersMS]

Extend escape analysis with a parallel "used" bit. A tracked local is used if it has at least one non-trivial use -- not a copy into another tracked local, and not a use the allocation satisfies statically (compare against null/zero, NULLCHECK, ARR_LENGTH, BOUNDS_CHECK, discarded value, LCL_ADDR as a store destination, non-GC IND/BLK). The relation is closed over the connection graph the same way escapes are.

Locals that neither escape nor are used have no externally visible effect. Leave them as heap allocations with reason "[unused]"; the unreferenced helper call is then removed by later liveness/DCE. Stack- allocating these locals instead produces a dead temp whose initialization stores survive, costing code size.

[AndyAyersMS]

If we have a dead un-escaping allocation, we're usually better off leaving it as is, rather than stack allocating it and hoping we can clean up the resulting stores.

This addresses some of the regressions seen in #128513.

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[Copilot]

Pull request overview

This PR extends ObjectAllocator’s escape/connection-graph analysis with a parallel “used” attribute and uses it to avoid stack-allocating allocation temps that have no non-trivial uses, leaving them as heap allocations (with a diagnostic reason) so later optimization can eliminate the now-dead allocation path.

Changes:

• Add m_DefinitelyUsedPointers tracking plus Is*Used queries and MarkIndexAsUsed.
• Seed “used” from the computed escape set and propagate “used” backwards through the existing connection graph via a generalized closure routine.
• Gate stack allocation in CanAllocateLclVarOnStack on “used” and classify certain parent-stack patterns as “trivial” uses in AnalyzeParentStack.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/coreclr/jit/objectalloc.h	Adds the “used” bitvector state and inline query helpers.
src/coreclr/jit/objectalloc.cpp	Implements “used” marking/closure and uses it to block stack allocation for unused allocation temps.

Comments suppressed due to low confidence (1)

src/coreclr/jit/objectalloc.cpp:2048

• In AnalyzeParentStack, the enumerator-copy propagation block guarded by if (isCopy) looks effectively dead: isCopy is set to false at the top of each loop iteration and is not set back to true in the GT_STORE_LCL_VAR case, so this condition will never be true here. If the intent is to treat a direct local->local store (possibly through BOX) as a copy, this likely needs to test wasCopy (the pre-reset value) or otherwise update isCopy before the check; otherwise consider removing this block to avoid misleading future maintenance.

                // If the source of this store is an enumerator local,
                // then the dest also becomes an enumerator local.
                //
                if (isCopy)
                {
                    CheckForEnumeratorUse(lclNum, dstLclNum);
                }