UefiCpuPkg/MpInitLib: don't shadow the microcode patch twice.

UefiCpuPkg/MpInitLib: don't shadow the microcode patch twice.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1982

MpInitLibInitialize in MpLib.c will be invoked on both PEI and DXE
CPU code, MicrocodeDetect would be performed twice and copy
Microcode from flash to memory twice as well, which consider as
duplicate work to lead longer boot time.
This patch just use microcode memory copied in PEI phase if exist.

Signed-off-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>