Also simplified ia32 KeyedStoreStubCompiler::GenerateStoreExternalArray a bit.

BUG=v8:2110
TEST=mjsunit/regress/regress-2110

Review URL: https://chromiumcodereview.appspot.com/10260011

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11472 ce2b1a6d-e550-0410-aec6-3dcde31c8c00