Implement representation selection as part of SimplifiedLowering. Representation selection also
requires inserting explicit representation change nodes to be inserted in the graph. Such nodes
are pure, but also need to be lowered to machine operators. They need to be scheduled first, to
determine the control input for any branches inside.

This CL requires extensive testing. More tests to follow.

R=rossberg@chromium.org
BUG=

Review URL: https://codereview.chromium.org/425003004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23026 ce2b1a6d-e550-0410-aec6-3dcde31c8c00