Tweak liveness when instructions are used in environments.

Instructions remain live when debuggable, but only instructions
with object types remain live when non-debuggable.

Enable StackVisitor::GetThisObject for optimizing.

Change-Id: Id87b2cbf33a02450059acc9993995782e5f28987
diff --git a/compiler/optimizing/ssa_builder.h b/compiler/optimizing/ssa_builder.h
index b414fb2..24dc449 100644
--- a/compiler/optimizing/ssa_builder.h
+++ b/compiler/optimizing/ssa_builder.h
@@ -33,7 +33,9 @@
  *
  * (a) Dex registers that do not require merging (that is, they do not
  *     have different values at a join block) are available to all their
- *     environment uses.
+ *     environment uses. Note that it does not imply the instruction will
+ *     have a physical location after register allocation. See the
+ *     SsaLivenessAnalysis phase.
  *
  * (b) Dex registers that require merging, and the merging gives
  *     incompatible types, will be killed for environment uses of that merge.