Linear scan: Use FirstUse instead of FirstRegisterUse.

This is in preparation for introducing synthesized used at back edges.

Change-Id: Ie28d6725d2dde982cf2137f2110daabcbab9f789
diff --git a/compiler/optimizing/nodes.h b/compiler/optimizing/nodes.h
index 649038b..44931c6 100644
--- a/compiler/optimizing/nodes.h
+++ b/compiler/optimizing/nodes.h
@@ -2162,7 +2162,7 @@
 
   uint32_t GetDexMethodIndex() const { return dex_method_index_; }
 
-  Intrinsics GetIntrinsic() {
+  Intrinsics GetIntrinsic() const {
     return intrinsic_;
   }