UefiCpuPkg PiSmmCpuDxeSmm: Reduce SMRAM consumption in CpuS3.c
    
    This patch makes two refinements to reduce SMRAM consumption in CpuS3.c.
    1. Only do CopyRegisterTable() when register table is not empty,
      IsRegisterTableEmpty() is created to check whether the register table
      is empty or not.
    
      Take empty PreSmmInitRegisterTable as example, about 24K SMRAM consumption
      could be reduced when mAcpiCpuData.NumberOfCpus=1024.
      sizeof (CPU_REGISTER_TABLE) = 24
      mAcpiCpuData.NumberOfCpus = 1024 = 1K
      mAcpiCpuData.NumberOfCpus * sizeof (CPU_REGISTER_TABLE) = 24K
    
    2. Only copy table entries buffer instead of whole buffer.
      AllocatedSize in SourceRegisterTableList is the whole buffer size.
      Actually, only the table entries buffer needs to be copied, and the size
      is TableLength * sizeof (CPU_REGISTER_TABLE_ENTRY).
    
      Take AllocatedSize=0x1000=4096, TableLength=100 and NumberOfCpus=1024 as example,
      about 1696K SMRAM consumption could be reduced.
      sizeof (CPU_REGISTER_TABLE_ENTRY) = 24
      TableLength = 100
      TableLength * sizeof (CPU_REGISTER_TABLE_ENTRY) = 2400
      AllocatedSize = 0x1000 = 4096
      AllocatedSize - TableLength * sizeof (CPU_REGISTER_TABLE_ENTRY) = 4096 - 2400 = 1696
      NumberOfCpus = 1024 = 1K
      NumberOfCpus * (AllocatedSize - TableLength * sizeof (CPU_REGISTER_TABLE_ENTRY)) = 1696K
    
    This patch also corrects the CopyRegisterTable() function description.
    
    Signed-off-by: Star Zeng <star.zeng@intel.com>
    Reviewed-by: Ray Ni <ray.ni@intel.com>
    Reviewed-by: Laszlo Ersek <lersek@redhat.com>
    Cc: Ray Ni <ray.ni@intel.com>
    Cc: Eric Dong <eric.dong@intel.com>
    Cc: Laszlo Ersek <lersek@redhat.com>
    Message-Id: <20210111015419.28368-1-star.zeng@intel.com>