This should solve the problem with missing checkpoints after JSToNumber
(PlainPrimitiveToNumber is marked no-write, so the frame-state
propagation should see through it.)

Unfortunately, this also duplicates the word32- and float64-truncation
magic that we have for JSToNumber in "simplified lowering".

Review-Url: https://codereview.chromium.org/2059653002
Cr-Commit-Position: refs/heads/master@{#36881}