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
diff --git a/compiler/utils/mips64/assembler_mips64.h b/compiler/utils/mips64/assembler_mips64.h
index c170313..d083eb4 100644
--- a/compiler/utils/mips64/assembler_mips64.h
+++ b/compiler/utils/mips64/assembler_mips64.h
@@ -46,6 +46,20 @@
   kStoreDoubleword
 };
 
+// Used to test the values returned by ClassS/ClassD.
+enum FPClassMaskType {
+  kSignalingNaN      = 0x001,
+  kQuietNaN          = 0x002,
+  kNegativeInfinity  = 0x004,
+  kNegativeNormal    = 0x008,
+  kNegativeSubnormal = 0x010,
+  kNegativeZero      = 0x020,
+  kPositiveInfinity  = 0x040,
+  kPositiveNormal    = 0x080,
+  kPositiveSubnormal = 0x100,
+  kPositiveZero      = 0x200,
+};
+
 class Mips64Assembler FINAL : public Assembler {
  public:
   Mips64Assembler() {}