Automatically anchor regexes beginning with .*

[stephentoub]

Avoid unnecessary backtracking by adding an implicit ^ when

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

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

    private readonly Regex _regex = new Regex(@".*text", RegexOptions.Compiled);

    [Benchmark] public bool IsNotMatch() => _regex.IsMatch("this is a test to see how well this does when trying to match this tex");
}

Method	Toolchain	Mean	Error	StdDev	Ratio	Gen 0	Gen 1	Gen 2	Allocated
IsNotMatch	\master\corerun.exe	10.796 us	0.0597 us	0.0498 us	1.00	-	-	-	-
IsNotMatch	\pr\corerun.exe	1.131 us	0.0008 us	0.0007 us	0.10	-	-	-	-

Without whitespace: https://github.com/dotnet/runtime/pull/1706/files?w=1

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

[stephentoub]

Thanks for reviewing, @danmosemsft.