[turbofan] Remove unused diamonds during control reduction.

[turbofan] Remove unused diamonds during control reduction.

A diamond is unused if the Merge node has no Phi/EffectPhi uses, exactly
two inputs, one IfTrue and one IfFalse, which have the same Branch
control input and no other uses except for the Merge. In this case the
diamond can safely be removed.

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/1000883003

Cr-Commit-Position: refs/heads/master@{#27148}