JIT: Physical promotion enhancements

[jakobbotsch]

This issue captures a list of potential enhancements to physical promotion, in no particular order.

CQ

• Decomposition via arithmetic
• Share backing storage when promoted locals are stack spilled
• Allow small-typed mismatches (JIT: Physical promotion bails on signedness mismatch on small types #104898)
• Support some bitcasts
• Partial lifetime promotion
• Allow promoted struct parameters to stay in registers (JIT: [experiment] Add an explicit IR representation for parameter definitions #92026)
• Allow promoted struct returns to stay in registers
• Custom class layouts with GC pointers (for some efficient decomposed copies) (JIT should support custom ClassLayout instances with GC refs #103362)
• Readback/writeback resolution
• Full support for GetElemet/WithElement for SIMDs (potentially as a separate pass)
• Assignment decomposition for GetElement/WithElement (JIT: Support decomposition into WithElement/GetElement in physical promotion #87554)
• Field support for LCL_ADDR propagation (JIT: suboptimal generated code for struct overlapped field manipulation #69254)

Throughput

• Stop tracking accesses early when we know we won't promote

Related issues

• JIT: suboptimal generated code for struct overlapped field manipulation #69254
• Regressions due to QJFL and OSR  #67594 (comment)

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[jakobbotsch]

A case from @hez2010 where we don't get good codegen on SysV today because some struct fields do not map cleanly to parameter registers:

public class Program
{
    public static void Main()
    {
        GetDistance<float>(default, default);
    }

    record struct Point<T>(T X, T Y) where T : INumberBase<T>, IRootFunctions<T>;

    [MethodImpl(MethodImplOptions.NoInlining)]
    static T GetDistance<T>(Point<T> p1, Point<T> p2) where T : INumber<T>, IRootFunctions<T>
    {
        T dx = p1.X - p2.X;
        T dy = p1.Y - p2.Y;
        return T.Sqrt(dx * dx + dy * dy);
    }
}