UefiCpuPkg/MtrrLib: Fix an assertion bug

UefiCpuPkg/MtrrLib: Fix an assertion bug

0                     40          f0         100
+---WT--+--UC--+--WT--+-----WB----+----UC----+

When calculating the shortest path from 0 to 100, the
MtrrLibCalculateLeastMtrrs() is called to update the
Vertices.Previous.
When calculating the shortest path from 0 to 40,
MtrrLibCalculateLeastMtrrs() is called recursively to update the
Vertices.Previous.
The second call corrupt the Previous value that will be used
later.
The patch removes the code that corrupts Previous.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>