MemoryExtensions.Replace(Span<T>, T, T) implemented (#76337)

MemoryExtensions.Replace(Span<T>, T, T) implemented (#76337)

* Defined API

* Tests

* Scalar implementation

* Use EqualityComparer<T>.Default instead

* Delegation to SpanHelpers.Replace

* ReplaceValueType implemented

* Use ushort instead of short, as it doesn't sign-extend for broadcast and in the scalar loop

* Forward string.Replace(char, char) to SpanHelpers.ReplaceValueType

* Process remainder vectorized only when not done already and with max width available

* Split into inlineable scalar path and non-inlineable vectorized path

* Replaced open coded loops with Replace

* Don't use EqualityComparer<T>.Default

Cf. https://github.com/dotnet/runtime/pull/76337#discussion_r982886319

* Remove guards for remainder

Cf. https://github.com/dotnet/runtime/pull/76337#discussion_r983448480

* Don't split method into scalar and vectorized and don't force inlining of scalar-part

* Fixed assert

ReplaceValueType is called from string.Replace(char, char) so the Debug.Assert was on wrong position, as at entry to method non accelerated platforms are allowed to call it.

* Better handling of remainder from the vectorized loop(s)

Intentionally leave one iteration off, as the remaining elements are done vectorized anyway. This eliminates the less probable case (cf. https://github.com/dotnet/runtime/pull/76337#discussion_r983448480) that the last vector is done twice.

* PR feedback