[experiment] fold definitely never instanceof casts

[EgorBo]

just a ci test to get diffs.

Example of what it folds:

static void Foo(object o)
{
    if (o is Test)
    {
        if (o is int x)
        {
            Console.WriteLine(x);
        }
    }
}

to no-op

[EgorBo]

@MihuBot -nuget

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[Copilot]

Pull request overview

This PR adds a JIT assertion-propagation optimization to fold exact type tests (method-table comparisons) to constants when existing subtype/exact-type assertions prove the object can never be exactly the tested type, including across PHI reaching assertions.

Changes:

• Introduces Compiler::optAssertionVNIsNotExactType to infer “never exactly this type” from subtype/exact-type assertions and PHI-reaching assertions.
• Uses the new helper in optAssertionPropGlobal_RelOp to fold IND(obj) ==/!= TypeHandle into false/true when provable.
• Adds the helper declaration to compiler.h.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/coreclr/jit/compiler.h	Declares the new optAssertionVNIsNotExactType assertion-prop helper.
src/coreclr/jit/assertionprop.cpp	Implements the helper and adds a new folding case in global relop assertion propagation for exact type tests.