Moving the remaining Vector128/256 helper intrinsics to be implemented using other...

Moving the remaining Vector128/256 helper intrinsics to be implemented using other intrinsics (dotnet/coreclr#21451)

* Updating Vector256.Create(V128, V128) to be implemented using other intrinsics

* Updating Vector128.Create(T, ...) and Vector256.Create(T, ...) to be implemented using other intrinsics

* Renaming CreateSoftware to SoftwareFallback and adding clarifying comments to the helper method code

* Fixing Vector128.Create(int, int, int, int) to properly consume the elements for the Sse2 codepath

Commit migrated from https://github.com/dotnet/coreclr/commit/aaa00748a9029dc23a3b5455e81a3133d91afe56