Represent cached generic context as a local

[AndyAyersMS]

Replace the anonymous generic-context frame offset with an early-created internal local while preserving existing reporting, OSR, and tailcall behavior.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

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[Copilot]

Pull request overview

This PR replaces the standalone “cached generic context” frame offset bookkeeping with an early-created internal local (lvaCachedGenericContextArg) whose stack offset is assigned during frame layout, aiming to preserve existing reporting (GC), OSR, and tailcall behavior.

Changes:

• Introduce an internal temp local to represent the cached generic context argument / reportable “this” slot, instead of tracking a raw frame offset.
• Update frame layout/refcount logic to (a) conditionally keep the local dormant/off-frame, and (b) assign/propagate its virtual stack offset (including OSR reuse of the root frame slot).
• Redirect lvaCachedGenericContextArgOffset() to read from the local’s LclVarDsc stack offset.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/coreclr/jit/lclvars.cpp	Creates and manages the new internal local; updates refcount and frame-layout paths to use its stack offset.
src/coreclr/jit/compiler.hpp	Makes lvaCachedGenericContextArgOffset() return the new local’s stack offset (with an assert).
src/coreclr/jit/compiler.h	Replaces the cached offset field with a cached-local var number initialized to BAD_VAR_NUM.

Suppressed comments (1)

src/coreclr/jit/lclvars.cpp:5110

• On JIT32_GCENCODER, lvaKeepAliveAndReportThis() can be true (e.g., synchronized instance methods), but this block is currently compiled out. That leaves lvaCachedGenericContextArg.lvOnFrame set (in lvaMarkLocalVars) without ever assigning a stack offset, and the later locals-allocation loop also skips lvaCachedGenericContextArg, so lvaCachedGenericContextArgOffset() can end up returning BAD_STK_OFFS when GC encoding asks for the reportable slot.

#ifndef JIT32_GCENCODER
    else if (lvaKeepAliveAndReportThis())
    {
        bool canUseExistingSlot = false;
        if (opts.IsOSR())