Expand an optimization for short values to IgnoreCase in single-value SearchValues<string>

[MihaZupan]

For values length 2 or 3, we have a special case where we don't verify matches at all if we know that false positives are impossible when all the anchors match.

runtime/src/libraries/System.Private.CoreLib/src/System/SearchValues/Strings/SingleStringSearchValuesThreeChars.cs

Lines 362 to 366 in ba9b3ba

	// If the value is short (!TValueLength.AtLeast4Chars => 2 or 3 characters), the anchors already represent the whole value.
	// With case-sensitive comparisons, we've therefore already confirmed the match, so we can skip doing so here.
	// With case-insensitive comparisons, we applied a mask to the input, so while the anchors likely matched, we can't be sure.
	if ((typeof(TCaseSensitivity) == typeof(CaseSensitive) && !TValueLength.AtLeast4Chars) ||
	TCaseSensitivity.Equals<TValueLength>(ref matchRef, _value))

We were using this trick with case-sensitive comparisons, but it's actually also valid if we know that all the characters are ASCII letters, so this PR extends the optimization to apply to those values as well.

---

public class EarlyMatches
{
    private const string Needle = "the";
    private static readonly string s_haystack = string.Concat(Enumerable.Repeat(Needle, 10_000));
    private static readonly SearchValues<string> s_values = SearchValues.Create([Needle], StringComparison.OrdinalIgnoreCase);

    [Benchmark]
    public int Count()
    {
        int count = 0;
        ReadOnlySpan<char> haystack = s_haystack;
        while (true)
        {
            int pos = haystack.IndexOfAny(s_values);
            if (pos < 0) break;
            count++;
            haystack = haystack.Slice(Needle.Length);
        }
        return count;
    }
}

Method	Toolchain	Mean	Error	Ratio	Code Size
Count	main	29.41 us	0.517 us	1.00	946 B
Count	pr	23.30 us	0.269 us	0.79	726 B

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