UefiCpuPkg/CpuMpPei: Skip microcode check/load if it has been loaded

UefiCpuPkg/CpuMpPei: Skip microcode check/load if it has been loaded

Actually, there is only one microcode region in platform. If microcode has been
loaded, its signature will not be zero and should be loaded successfully.
We needn't to check microcode region and load microcode again. This update is to
skip checking/loading microcode if current microcode signature is not zero.

Cc: Michael Kinney <michael.d.kinney@intel.com>
Cc: Feng Tian <feng.tian@intel.com>
Cc: Giri P Mudusuru <giri.p.mudusuru@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Giri P Mudusuru <giri.p.mudusuru@intel.com>