Quick compiler: Fix liveness tracking

Rework temp register liveness tracking to play nicely with aliased
physical registers, and re-enable liveness tracking optimization.

Add a pair of x86 utility routines that act like UpdateLoc(),
but only show in-register live temps if they are of the expected
register class.

Change-Id: I92779e0da2554689103e7488025be281f1a58989
diff --git a/compiler/dex/frontend.cc b/compiler/dex/frontend.cc
index 89c642d..d9d392f 100644
--- a/compiler/dex/frontend.cc
+++ b/compiler/dex/frontend.cc
@@ -46,7 +46,7 @@
   // (1 << kNullCheckElimination) |
   // (1 << kClassInitCheckElimination) |
   // (1 << kPromoteRegs) |
-  (1 << kTrackLiveTemps) |        // FIXME: disable until liveness issue fixed.
+  // (1 << kTrackLiveTemps) |
   // (1 << kSafeOptimizations) |
   // (1 << kBBOpt) |
   // (1 << kMatch) |