Suppress canonicalization of Unbox() instruction that can deoptimize.

Suppress canonicalization of instructions that have unsatisfied input representations.

BUG=
R=fschneider@google.com

Review URL: https://codereview.chromium.org//721773003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41693 260f80e4-7a28-3924-810f-c04153c831b5