Use SearchValues<string> in Regex for single string searches

[stephentoub]

#88400 updates regex to use SearchValues<string> for multi-substring search. But SearchValues<string> is also able to optimize single-substring search beyond what IndexOf(span, StringComparison.Ordinal{IgnoreCase}) does, as it can precompute and analyze more. We should update the Regex source generator (and possibly the RegexCompiler, although the equation for it is a bit different, as there's a little more overhead for the compiler to access a cached SearchValues<string>) to use SearchValues<string> for some of the places where it searches for a string, in particular the next-starting-position search.

[ghost]

Tagging subscribers to this area: @dotnet/area-system-text-regularexpressions
See info in area-owners.md if you want to be subscribed.

Issue Details

---

#88400 updates regex to use SearchValues<string> for multi-substring search. But SearchValues<string> is also able to optimize single-substring search beyond what IndexOf(span, StringComparison.Ordinal{IgnoreCase}) does, as it can precompute and analyze more. We should update the Regex source generator (and possibly the RegexCompiler, although the equation for it is a bit different, as there's a little more overhead for the compiler to access a cached SearchValues<string>) to use SearchValues<string> for some of the places where it searches for a string, in particular the next-starting-position search.

Author:	stephentoub
Assignees:	-
Labels:	area-System.Text.RegularExpressions, tenet-performance
Milestone:	9.0.0

[stephentoub]

This is done