Use dex cache from compilation unit in RTP.

Avoid calling the costly ClassLinker::FindDexCache() from
reference type propagation when the dex cache from the
compilation unit will do, i.e. almost always. Compiling
the Nexus 5 boot image on host under perf(1) shows that
the FindDexCache() hits drop from about 0.2% to almost
nothing, though enabling inlining for the boot image will
increase it a bit to 0.03% due to unavoidable calls from
the inliner.

Also clean up the ScopedObjectAccess usage a bit.

Bug: 28173563

Cherry-picked the "revert-revert"
    (cherry picked from commit 456307a47336e3d6576ed6d8563b67573a4238d3)
and squashed two subsequent fixes
    Fix RTP to hold mutator lock while using raw mirror pointers.
    (cherry picked from commit 62977ff198deb673a6990202a2fb8b993217c57c)
    Fix reference_type_propagation_test.
    (cherry picked from commit 5eed0c5d27f091c952704f652cd77c4e3833ad88)

Change-Id: Ia944452d7ab26aed963832a9346df363743a419f
diff --git a/compiler/optimizing/ssa_builder.cc b/compiler/optimizing/ssa_builder.cc
index f935585..35aa1b1 100644
--- a/compiler/optimizing/ssa_builder.cc
+++ b/compiler/optimizing/ssa_builder.cc
@@ -503,7 +503,7 @@
 
   // 4) Compute type of reference type instructions. The pass assumes that
   // NullConstant has been fixed up.
-  ReferenceTypePropagation(graph_, handles_, /* is_first_run */ true).Run();
+  ReferenceTypePropagation(graph_, dex_cache_, handles_, /* is_first_run */ true).Run();
 
   // 5) HInstructionBuilder duplicated ArrayGet instructions with ambiguous type
   // (int/float or long/double) and marked ArraySets with ambiguous input type.