The PlainPrimitiveToNumber operator performs a superset of the operations
previously performed by the BooleanToNumber and StringToNumber operators,
so we can just use the special lowering rules for PlainPrimitiveToNumber
based on the input type and get rid of the specialized operators.

R=jarin@chromium.org

Review-Url: https://codereview.chromium.org/2139183002
Cr-Commit-Position: refs/heads/master@{#37669}