The original arithmetic lowering was introducing some
unused mov instructions from legalization (e.g., the upper
part of shift num bits -- which should be 0 anyway), and
div helper calls don't actually use the legalized
parameters (handled separately by lowerCall). These unused
instructions cause the Om1 allocator to assert that LRBegin
exists but LREnd does not.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4076
R=kschimpf@google.com

Review URL: https://codereview.chromium.org/1210073017.