Optimize Dictionary.FindValue for constant keys (no constant folding)

[EgorBo]

Since I was not able to easily workaround the arg is address exposed caused by uint hashCode = (uint)key.GetHashCode(); line (which is ldarga + constrained. callvirt) which in its turns breaks the inliner-level constant propagation, let's see if this helps.
Also, remove string.EqualsOrdinalIgnoreCase - it was not unrolling friendly.

[EgorBo]

@MihuBot

[EgorBo]

Blocked on #117431

[EgorBo]

@EgorBot -amd -intel -arm

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

BenchmarkSwitcher.FromAssembly(typeof(Program).Assembly).Run(args);

public class StringDictionaryDefault
{
    private static readonly Dictionary<string, int> StringDictionary = new()
    {
        { "DOTNET_ThreadPool_UnfairSemaphoreSpinLimit" + 1, 1 },
        { "Hello World" + 2, 2 },
        { "aa" + 3, 3 },
    };

    [Benchmark] public int LongKey() => StringDictionary["DOTNET_ThreadPool_UnfairSemaphoreSpinLimit1"];
    [Benchmark] public int MediumKey() => StringDictionary["Hello World2"];
    [Benchmark] public int ShortKey() => StringDictionary["aa3"];
}

public class StringDictionaryIgnoreCase
{
    private static readonly Dictionary<string, int> StringDictionary = new(StringComparer.OrdinalIgnoreCase)
    {
        { "DOTNET_ThreadPool_UnfairSemaphoreSpinLimit" + 1, 1 },
        { "Hello World" + 2, 2 },
        { "aa" + 3, 3 }
    };

    [Benchmark] public int LongKey_IgnoreCase() => StringDictionary["DOTNET_ThreadPool_UnfairSemaphoreSpinLimit1"];
    [Benchmark] public int MediumKey_IgnoreCase() => StringDictionary["Hello World2"];
    [Benchmark] public int ShortKey_IgnoreCase() => StringDictionary["aa3"];
}

public class StringDictionary_SameRef
{
    private static readonly Dictionary<string, int> StringDictionary = new()
    {
        { "DOTNET_ThreadPool_UnfairSemaphoreSpinLimit", 1 },
        { "Hello World", 2 },
        { "aa", 3 }
    };

    [Benchmark] public int LongKey_SameRef() => StringDictionary["DOTNET_ThreadPool_UnfairSemaphoreSpinLimit"];
    [Benchmark] public int MediumKey_SameRef() => StringDictionary["Hello World"];
    [Benchmark] public int ShortKey_SameRef() => StringDictionary["aa"];
}

[EgorBo]

PTAL @stephentoub essentially what this PR does - it enables unrolling for this Equals call (for both Ordinal and OrdinalIgnoreCase comparers) when Dynamic PGO decides to devirtualize those and the key is a string literal.

Benchmarks.

No impact on value-type keys.

[EgorBo]

/ba-g "android-x64 timed out"