[llvm][tools][opt-viewer] Put back missing function

Fixes 35e1a2f

itervalues is in fact used in opt-viewer.py.

Author: DavidSpickett

llvm/tools/opt-viewer/optrecord.py

@@ -25,6 +25,10 @@
 import optpmap
 
 
+def itervalues(d):
+    return iter(d.values())
+
+
 def iteritems(d):
     return iter(d.items())