Vectorize String.IndexOf(char) and String.LastIndexOf(char) (#16392)
Vectorize String.IndexOf(char) and String.LastIndexOf(char) (#16392)

* Vectorize String.IndexOf(char) using the same algorithm as SpanHelpers
IndexOf(byte).

* Respond to feedback.

* Vectorize String.LastIndexOf

Clean up IndexOf vectorization.

ruben-ayrapetyan/coreclr,wtgodbe/coreclr,mmitche/coreclr,mmitche/coreclr,krk/coreclr,cshung/coreclr,cshung/coreclr,wtgodbe/coreclr,ruben-ayrapetyan/coreclr,poizan42/coreclr,krk/coreclr,poizan42/coreclr,cshung/coreclr,ruben-ayrapetyan/coreclr,cshung/coreclr,poizan42/coreclr,mmitche/coreclr,poizan42/coreclr,mmitche/coreclr,krk/coreclr,wtgodbe/coreclr,cshung/coreclr,mmitche/coreclr,krk/coreclr,krk/coreclr,krk/coreclr,ruben-ayrapetyan/coreclr,ruben-ayrapetyan/coreclr,cshung/coreclr,wtgodbe/coreclr,ruben-ayrapetyan/coreclr,wtgodbe/coreclr,poizan42/coreclr,mmitche/coreclr,wtgodbe/coreclr,poizan42/coreclr