Automatically anchor regexes beginning with .*

[stephentoub]

Avoid unnecessary backtracking by adding an implicit ^ when

usingBenchmarkDotNet.Attributes;usingBenchmarkDotNet.Diagnosers;usingBenchmarkDotNet.Running;usingSystem.Text.RegularExpressions;[MemoryDiagnoser]publicclassProgram{staticvoidMain(string[]args)=>BenchmarkSwitcher.FromAssemblies(new[]{typeof(Program).Assembly}).Run(args);privatereadonlyRegex_regex=newRegex(@".*text",RegexOptions.Compiled);[Benchmark]publicboolIsNotMatch()=>_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.