Fix HCheckValue::Canonicalize wrt uninitialized HConstant unique.

R=titzer@chromium.org
BUG=348280
LOG=N

Review URL: https://codereview.chromium.org/183383006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19642 ce2b1a6d-e550-0410-aec6-3dcde31c8c00