[turbofan] Canonicalize return sequence for JSFunctions.

[turbofan] Canonicalize return sequence for JSFunctions.

This optimization is already implemented in fullcodegen, and
basically makes sure that we do not unecessarily blow up the
code with duplicated return sequences everywhere.

R=danno@chromium.org

Review URL: https://codereview.chromium.org/1211373002

Cr-Commit-Position: refs/heads/master@{#29315}