Implemented BitCount as an intrinsic. With unit test.

Rationale:
Recognizing this important operation as an intrinsic has
various advantages:
(1) having the no-side-effects/no-throw allows for
    much more GVN/LICM/BCE.
(2) Some architectures, like x86_64, provide direct
    support for this operation.

Performance improvements on X86_64:
CheckersEvalBench (32-bit bitboard): 27,210KNS -> 36,798KNS  =  + 35%
ReversiEvalBench  (64-bit bitboard): 52,562KNS -> 89,086KNS  =  + 69%

Change-Id: I65d549b0469b7909b12c6611cdc34a8640a5751f
diff --git a/compiler/optimizing/intrinsics_mips64.cc b/compiler/optimizing/intrinsics_mips64.cc
index 8b45ea7..ac969e3 100644
--- a/compiler/optimizing/intrinsics_mips64.cc
+++ b/compiler/optimizing/intrinsics_mips64.cc
@@ -1724,6 +1724,9 @@
 void IntrinsicCodeGeneratorMIPS64::Visit ## Name(HInvoke* invoke ATTRIBUTE_UNUSED) {    \
 }
 
+UNIMPLEMENTED_INTRINSIC(IntegerBitCount)
+UNIMPLEMENTED_INTRINSIC(LongBitCount)
+
 UNIMPLEMENTED_INTRINSIC(MathRoundDouble)
 UNIMPLEMENTED_INTRINSIC(MathRoundFloat)