UefiCpuPkg/MtrrLib: refine MtrrLibProgramFixedMtrr()
UefiCpuPkg/MtrrLib: refine MtrrLibProgramFixedMtrr()

The patch replaces some if-checks with assertions because
they are impossible to happen.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ruiyu Ni <9fa3a6894a36bca79e6a72ad65af5a5702efecd1@intel.com>
Cc: Michael D Kinney <fd20bc543a9f65bb633fac7d08a403c1f1c8eb6c@intel.com>
Cc: Eric Dong <fa9eeec52367040ac98013bc4b2913db170588f8@intel.com>
Reviewed-by: Jiewen Yao <364a90bfebd1f362ebb7b48e4bf8ec010adef203@intel.com>

MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2