Remember required register kind when creating artificial virtual register.

src/data-flow.h

Index: src/data-flow.h
diff --git a/src/data-flow.h b/src/data-flow.h
index 6e2230c65ea5d3b2051a766aa62347c914cf61b9..79d760f5a4267433f41c7847482ac9e3acefcdf7 100644
--- a/src/data-flow.h
+++ b/src/data-flow.h
@@ -112,10 +112,13 @@ class BitVector: public ZoneObject {
   }
 
   void CopyFrom(const BitVector& other) {
-    ASSERT(other.length() == length());
-    for (int i = 0; i < data_length_; i++) {
+    ASSERT(other.length() <= length());
+    for (int i = 0; i < other.data_length_; i++) {
       data_[i] = other.data_[i];
     }
+    for (int i = other.data_length_; i < data_length_; i++) {
+      data_[i] = 0;
+    }
   }
 
   bool Contains(int i) const {