Fix major System.Memory performance regression on .NET Framework x86 (#28179)

Fix major System.Memory performance regression on .NET Framework x86 (#28179)

* Use Unsafe.AddByteOffset instead of Unsafe.Add where possible

* Use pointer for NUint backing field

* Delete unnecessary unchecked scopes, fixed comments

* Avoid unnecessary overflow checks when converting from IntPtr

* Switch SequenceEqual to IntPtr pointer arithmetic

This is fixing 10x regression introduced by use of NUint

* Delete NUint operators that are no longer necessary

* Fix CoreCLR build break

Signed-off-by: dotnet-bot-corefx-mirror <dotnet-bot@microsoft.com>