Enable Regex compiler to be trimmed (dotnet/corefx#41075)

Enable Regex compiler to be trimmed (dotnet/corefx#41075)

Today an app that just does:
```C#
Console.WriteLine(Regex.IsMatch("12345", "0*[1-9][0-9]*"));
```
and is trimmed will end up publishing a 105K System.Text.RegularExpression.dll, including the regex compiler for when the RegexOptions.Compiled is used.  We can refactor the constructors such that the compiler only ends up getting rooted when one of the ctors that takes options is used.  After this PR, if you don't pass RegexOptions, the 105K drops to 85K.

Commit migrated from https://github.com/dotnet/corefx/commit/31a5bba0e79a5ae875f691d70ec3b9789691136d