Additionally force increment instruction to use int32 representation.

R=fschneider@google.com
BUG=http://crbug.com/115646
TEST=test/mjsunit/compiler/optimized-for-in.js

Review URL: https://chromiumcodereview.appspot.com/9463052

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10844 ce2b1a6d-e550-0410-aec6-3dcde31c8c00