HW intrinsics: Expose REP MOVSB/D in API to allow conscious use of ERMSB feature

[voinokin]

Please expose REP MOVSB/D in HW intrinsics API to allow use of ERMSB feature to copy memory blocks without use of SSE and later. obvious use case is - transfer of large memory areas, like set of VM pages (typically N x 4Kb for Windows).

Example of numbers obtained by benchmarking and some descriptive text can be found here:
https://stackoverflow.com/a/43574756

category:proposal
theme:hardware-intrinsics
skill-level:intermediate
cost:medium

[tannergooding]

@voinokin, given that this is a new API/ISA (and not part of the already approved ISAs) could you open an API proposal against CoreFX (following the recommended processes laid out here: https://github.com/dotnet/corefx/blob/master/Documentation/project-docs/api-review-process.md#steps)?