CpuPageTableLib: Refactor the logic

CpuPageTableLib: Refactor the logic

The patch replaces
  LinearAddress + Offset == RegionStart
with
  ((LinearAddress + Offset) & RegionMask) == 0

The replace should not cause any behavior change.

Because:
1. In first loop of while when LinearAddress + Offset == RegionStart,
  because the lower "BitStart" bits of RegionStart are all-zero,
  all lower "BitStart" bits of (LinearAddress + Offset) are all-zero.
  Because all lower "BitStart" bits of RegionMask is all-one and
  bits are all-zero, ((LinearAddress + Offset) & RegionMask) == 0.

2. In following loops of the while, even RegionStart is increased
  by RegionLength, the lower "BitStart" bits are still all-zero.
  So the two expressions still semantically equal to each other.

Signed-off-by: Ray Ni <ray.ni@intel.com>
Cc: Zhiguang Liu <zhiguang.liu@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>