Optimize newline handling for RegexOptions.Multiline

[stephentoub]

We previously didn't do any special handling of beginning-of-line anchors (^ when RegexOptions.Multiline is specified). This PR adds special handling for the anchor so that FindFirstChar will jump to the next newline as part of its processing.

As part of this, I also cleaned up some of the anchor handling code. The RegexPrefixAnalyzer only ever returns a single anchor, but the rest of the code was written such that it was expecting multiple anchors.

Example:
Finding all lines in Romeo and Juliet that contain the word "love":

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

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

    private readonly string _input = File.ReadAllText(@"C:\romeojuliet.txt");
    private Regex _regex;

    [Params(false, true)]
    public bool Compiled { get; set; }

    [GlobalSetup]
    public void Setup() => _regex = new Regex(@"^.*\blove\b.*$", RegexOptions.Multiline | (Compiled ? RegexOptions.Compiled : RegexOptions.None));

    [Benchmark]
    public int Count() => _regex.Matches(_input).Count;
}

Method	Toolchain	Compiled	Mean	Error	StdDev	Median	Ratio	RatioSD
Count	\netcore31\corerun.exe	False	21.184 ms	0.1938 ms	0.1619 ms	21.179 ms	2.15	0.05
Count	\master\corerun.exe	False	9.868 ms	0.2870 ms	0.2545 ms	9.784 ms	1.00	0.00
Count	\pr\corerun.exe	False	4.436 ms	0.0482 ms	0.0428 ms	4.445 ms	0.45	0.01
Count	\netcore31\corerun.exe	True	16.929 ms	0.3705 ms	1.0630 ms	16.122 ms	3.96	0.23
Count	\master\corerun.exe	True	4.292 ms	0.0415 ms	0.0388 ms	4.272 ms	1.00	0.00
Count	\pr\corerun.exe	True	2.167 ms	0.0045 ms	0.0040 ms	2.166 ms	0.50	0.00

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

[eerhardt]

Looks good to me. Nice improvement!

[pgovind]

Nice!!

[danmoseley]

Another case to handle if @jzabroski still plans to revive dotnet/corefx#41195

edit: I meant #1449

[jzabroski]

@danmosemsft I do plan to. Just got trapped under mountain of COVID-19 DR planning work. I expect to give it the old college try in about 2 weeks. Thanks for thinking of me and tagging me.

[danmoseley]

Sure @jzabroski.. we will get there eventually!