Add JavascriptNumber fastpath in JavascriptOperators::GetIndexTypeFromPrimitive
Add JavascriptNumber fastpath in JavascriptOperators::GetIndexTypeFromPrimitive

Mitigates OS: 17348829.

We recently reactivated the JS Built In implementation of indexOf. It seems there is some bad codegen in the JIT that is passing an unsigned integer as a double through this path, causing a slow ToString of the index integer and causing a significant perf hit.

Regardless of the JIT issue, there's no reason for us to do a ToString on a JavascriptNumber if we don't have to. Added a check to see if we can use it as an index, otherwise go through the ToString as usual.

Microsoft/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore