Change GenerateLoadStringLength to have two scratch registers on all platforms.  This is needed so that the LoadIC can pass the receiver in a register, and not have it overwritten, and also simplifies the code.
Review URL: http://codereview.chromium.org/596011

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3820 ce2b1a6d-e550-0410-aec6-3dcde31c8c00