Handlify DescriptorArray::Merge().

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/169363002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19410 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Cleanup the double field tracking in Hydrogen.

Use a dedicated BuildLoadNamedField() with PropertyAccessInfo,
similar to BuildStoreNamedField() for optimized graph building,
and a dedicated BuildLoadNamedField() for the code stubs, and
don't depend on FLAG_track_double_fields during code generation.

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/168583006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19411 ce2b1a6d-e550-0410-aec6-3dcde31c8c00