MIPS64: Code cleanup.

1. Add comment to explain logic for GenMinMax in intrinsics code.
2. Declare enumerated type for class.s/class.d mask values.
3. Change intrinsics code to use the enumerated values described in
   item 2.
4. Change "CLASS_MASK" to "kFPLeaveUnchanged" to match ART coding
   standards.

Change-Id: Ib1fe4b01515595b46e5f101e0082bb9bbcf0c688