Compile-time tuning

Specialized the dataflow iterators and did a few other minor tweaks.
Showing ~5% compile-time improvement in a single-threaded environment;
less in multi-threaded (presumably because we're blocked by something
else).

Change-Id: I2e2ed58d881414b9fc97e04cd0623e188259afd2
diff --git a/compiler/dex/quick/mir_to_lir-inl.h b/compiler/dex/quick/mir_to_lir-inl.h
index 440df2a..f9ec199 100644
--- a/compiler/dex/quick/mir_to_lir-inl.h
+++ b/compiler/dex/quick/mir_to_lir-inl.h
@@ -33,7 +33,12 @@
     p->def_end = NULL;
     if (p->pair) {
       p->pair = false;
-      Clobber(p->partner);
+      p = GetRegInfo(p->partner);
+      p->pair = false;
+      p->live = false;
+      p->s_reg = INVALID_SREG;
+      p->def_start = NULL;
+      p->def_end = NULL;
     }
   }
 }