Replace CountOneBits and __builtin_popcount with POPCOUNT.

Clean up utils.h, make some functions constexpr.

Change-Id: I2399100280cbce81c3c4f5765f0680c1ddcb5883
diff --git a/compiler/dex/quick/codegen_util.cc b/compiler/dex/quick/codegen_util.cc
index b030bb4..0596d4f 100644
--- a/compiler/dex/quick/codegen_util.cc
+++ b/compiler/dex/quick/codegen_util.cc
@@ -1018,8 +1018,8 @@
       vmap_encoder.PushBackUnsigned(fp_vmap_table_[i] + VmapTable::kEntryAdjustment);
     }
   } else {
-    DCHECK_EQ(__builtin_popcount(core_spill_mask_), 0);
-    DCHECK_EQ(__builtin_popcount(fp_spill_mask_), 0);
+    DCHECK_EQ(POPCOUNT(core_spill_mask_), 0);
+    DCHECK_EQ(POPCOUNT(fp_spill_mask_), 0);
     DCHECK_EQ(core_vmap_table_.size(), 0u);
     DCHECK_EQ(fp_vmap_table_.size(), 0u);
     vmap_encoder.PushBackUnsigned(0u);  // Size is 0.