Deduplicate stack masks

The stack masks repeat often enough so that it is worth deduplicating
them.

Oat size for a large app:
98143600 -> 96722288 (-1.44%)

Bug: 34621054

Test: test-art-host
Change-Id: If73d51e46066357049d5be2e406ae9a32b7ff1f4
diff --git a/runtime/memory_region.cc b/runtime/memory_region.cc
index b0ecab4..13cc5c9 100644
--- a/runtime/memory_region.cc
+++ b/runtime/memory_region.cc
@@ -29,8 +29,7 @@
   CHECK_GT(from.size(), 0U);
   CHECK_GE(this->size(), from.size());
   CHECK_LE(offset, this->size() - from.size());
-  memmove(reinterpret_cast<void*>(start() + offset),
-          from.pointer(), from.size());
+  memmove(reinterpret_cast<void*>(begin() + offset), from.pointer(), from.size());
 }
 
 void MemoryRegion::StoreBits(uintptr_t bit_offset, uint32_t value, size_t length) {