Minor induction variable analysis changes.

Rationale:
Few extra comments, added replacement of loop control functionality.
All in preparation of bigger loop optimizations CL.

Change-Id: Ie2243c3c8b57b2901654b05fb200e9d80a0cb641
Test: test-art
diff --git a/compiler/optimizing/induction_var_analysis.cc b/compiler/optimizing/induction_var_analysis.cc
index 129c2a9..c501ccf 100644
--- a/compiler/optimizing/induction_var_analysis.cc
+++ b/compiler/optimizing/induction_var_analysis.cc
@@ -714,10 +714,12 @@
     case kCondGE: op = kGE; break;
     default:      LOG(FATAL) << "CONDITION UNREACHABLE";
   }
+  // Associate trip count with control instruction, rather than the condition (even
+  // though it's its use) since former provides a convenient use-free placeholder.
+  HInstruction* control = loop->GetHeader()->GetLastInstruction();
   InductionInfo* taken_test = CreateInvariantOp(op, lower_expr, upper_expr);
-  AssignInfo(loop,
-             loop->GetHeader()->GetLastInstruction(),
-             CreateTripCount(tcKind, trip_count, taken_test, type));
+  DCHECK(control->IsIf());
+  AssignInfo(loop, control, CreateTripCount(tcKind, trip_count, taken_test, type));
 }
 
 bool HInductionVarAnalysis::IsTaken(InductionInfo* lower_expr,