Added support for truncating DoubleToIStub and reorganize the macro-assembler
dToI operations to do the fast-path inline and the slow path by calling the
stub.

BUG=
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/23129003

Patch from Ross McIlroy <rmcilroy@google.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16322 ce2b1a6d-e550-0410-aec6-3dcde31c8c00