Move varint encoding/decoding to BitMemoryReader/Writer.

Test: test-art-host-gtest-bit_memory_region_test
Change-Id: I0f7d7dfc9efe9df0e3e746b1741c46f06a0bd3b4
diff --git a/compiler/optimizing/stack_map_stream.cc b/compiler/optimizing/stack_map_stream.cc
index 3918b65..60ca61c 100644
--- a/compiler/optimizing/stack_map_stream.cc
+++ b/compiler/optimizing/stack_map_stream.cc
@@ -296,10 +296,10 @@
 
   ScopedArenaVector<uint8_t> buffer(allocator_->Adapter(kArenaAllocStackMapStream));
   BitMemoryWriter<ScopedArenaVector<uint8_t>> out(&buffer);
-  EncodeVarintBits(out, packed_frame_size_);
-  EncodeVarintBits(out, core_spill_mask_);
-  EncodeVarintBits(out, fp_spill_mask_);
-  EncodeVarintBits(out, num_dex_registers_);
+  out.WriteVarint(packed_frame_size_);
+  out.WriteVarint(core_spill_mask_);
+  out.WriteVarint(fp_spill_mask_);
+  out.WriteVarint(num_dex_registers_);
   EncodeTable(out, stack_maps_);
   EncodeTable(out, register_masks_);
   EncodeTable(out, stack_masks_);