[turbofan] Support lowering of ChangeFloat64ToTagged/ChangeTaggedToInt32.

Adds new ValueEffect operator to ensure proper scheduling of
AllocateHeapNumber call nodes.

Also includes some refactoring to reduce code duplication.

TEST=compiler-unittests
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/481903002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23175 ce2b1a6d-e550-0410-aec6-3dcde31c8c00