Save an instruction in VFP immediate creation by passing a scratch register.

BUG=none
TEST=none

Review URL: https://chromiumcodereview.appspot.com/10990024
Patch from Martyn Capewell <m.m.capewell@googlemail.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12605 ce2b1a6d-e550-0410-aec6-3dcde31c8c00