UefiCpuPkg: Clean up save state boundary checks and comments.

UefiCpuPkg: Clean up save state boundary checks and comments.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2956

In functions ReadSaveStateRegisterByIndex and WriteSaveStateRegister:
* check width > 4 instead of >= 4 when writing upper 32 bytes.
  - This improves the code but will not affect functionality.

Cc: Eric Dong <eric.dong@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>
Signed-off-by: Mark Wilson <Mark.Wilson@amd.com>