Use IndexOf for .* in RegexInterpreter/Compiler

[stephentoub]

The first commit primarily cleans up style in RegexInterpreter. As part of that there were a few small substantive changes:

• Stored the TextInfo rather than storing the CultureInfo and accessing the TextInfo virtual property on each call.
• Coalesced duplicate case blocks.
• Removed unnecessary resx string that should have been an assert.

The second commit uses IndexOf to implement Notoneloop{atomic} when we're left-to-right and case-sensitive (which is the typical case). This is most beneficial when someone uses a loop like .*, which will generally become [^\n]*, and thus we IndexOf('\n') to find the end of the greedy consumption. If the end is found within just a few characters, there's a tiny regression, due to the overhead of calling AsSpan(...).IndexOf, but when there's more than that, the gains can be substantial.

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System.Text.RegularExpressions;

[MemoryDiagnoser]
public class Program
{
    static void Main(string[] args) => BenchmarkSwitcher.FromAssemblies(new[] { typeof(Program).Assembly }).Run(args);

    [Params(RegexOptions.None, RegexOptions.Compiled)]
    public RegexOptions Options { get; set; }

    [Params(1, 2, 4, 8, 64, 128)]
    public int Length { get; set; }

    private Regex _regex;
    private string _matchInput;

    [GlobalSetup]
    public void Setup()
    {
        _regex = new Regex("\".*\"", Options);
        _matchInput = '"' + new string('a', Length) + '"';
    }

    [Benchmark] public bool IsMatch() => _regex.IsMatch(_matchInput);
}

Method	Toolchain	Options	Length	Mean	Error	StdDev	Ratio
IsMatch	\master\corerun.exe	None	1	89.75 ns	0.332 ns	0.294 ns	1.00
IsMatch	\pr\corerun.exe	None	1	91.28 ns	0.366 ns	0.306 ns	1.02
IsMatch	\master\corerun.exe	None	2	90.46 ns	0.124 ns	0.110 ns	1.00
IsMatch	\pr\corerun.exe	None	2	92.76 ns	0.386 ns	0.301 ns	1.03
IsMatch	\master\corerun.exe	None	4	97.74 ns	0.102 ns	0.086 ns	1.00
IsMatch	\pr\corerun.exe	None	4	93.63 ns	0.523 ns	0.489 ns	0.96
IsMatch	\master\corerun.exe	None	8	105.08 ns	0.711 ns	0.665 ns	1.00
IsMatch	\pr\corerun.exe	None	8	93.50 ns	0.242 ns	0.226 ns	0.89
IsMatch	\master\corerun.exe	None	64	227.09 ns	0.396 ns	0.370 ns	1.00
IsMatch	\pr\corerun.exe	None	64	97.02 ns	0.129 ns	0.114 ns	0.43
IsMatch	\master\corerun.exe	None	128	365.66 ns	3.756 ns	2.933 ns	1.00
IsMatch	\pr\corerun.exe	None	128	93.92 ns	0.696 ns	0.651 ns	0.26

Contributes to #1349
cc: @danmosemsft, @eerhardt, @ViktorHofer

[eerhardt]