Fix SHL when shift amount is zero for arm and mips.

BUG=v8:2817
R=titzer@chromium.org
TEST=octane

Review URL: https://chromiumcodereview.appspot.com/21358002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15987 ce2b1a6d-e550-0410-aec6-3dcde31c8c00