MdeModulePkg: Update PeiCore dispatcher to handle PEIM and PEI_CORE only

MdeModulePkg: Update PeiCore dispatcher to handle PEIM and PEI_CORE only

When PcdShadowPeimOnBoot is FALSE, they are not copied to memory and
execute from their original locations. Here, this policy should only
apply for PEIM and PEI_CORE, not for other file type, such as DXE_CORE.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Tested-by: Eugene Cohen <eugene@hp.com>