Cache stack map encoding

Operations on CodeInfo and StackMap objects repeatedly read encoding
information from the MemoryRegion. Since these are 3-bit-loads of
values that never change, caching them can measurably reduce compile
times.

According to benchmarks, this patch saves 1-3% on armv7, 2-4% on x86,
and 0-1% on x64.

Change-Id: I46b197513601325d8bab562cc80100c00ec28a3b
diff --git a/compiler/optimizing/stack_map_stream.h b/compiler/optimizing/stack_map_stream.h
index 0af983b..274d573 100644
--- a/compiler/optimizing/stack_map_stream.h
+++ b/compiler/optimizing/stack_map_stream.h
@@ -171,6 +171,7 @@
 
   StackMapEntry current_entry_;
   InlineInfoEntry current_inline_info_;
+  StackMapEncoding stack_map_encoding_;
   size_t stack_mask_size_;
   size_t inline_info_size_;
   size_t dex_register_maps_size_;