Bounds check after overlapping span length checks

[MihaZupan]

https://godbolt.org/z/3arPqT9oP

public static bool Foo(ReadOnlySpan<char> span)
{
    for (int i = 0; i < span.Length; i++)
    {
        if (span[i] == '%' && (uint)(i + 2) < (uint)span.Length)
        {
            if (span[i + 1] == 'F' && // Bounds check
                span[i + 2] == 'F')
            {
                return true;
            }
        }
    }
    
    return false;
}

(uint)i < (uint)span.Length && (uint)(i + 2) < (uint)span.Length implies that i + 1 is also in range.

E.g.

runtime/src/libraries/System.Private.Uri/src/System/Uri.cs

Lines 3682 to 3686 in 0ad494b

	if ((uint)(i + 2) >= (uint)uriString.Length ||
	colonOffset == 0 ||
	// Redundant checks to eliminate range checks below
	(uint)i >= (uint)uriString.Length ||
	(uint)(i + 1) >= (uint)uriString.Length)

or
https://github.com/MihaZupan/runtime/blob/299818ab507ff37b0ef74918fdc35463b7e2d5e1/src/libraries/System.Private.Uri/src/System/UriHelper.cs#L321-L325 from #121261

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