IsData: generate strict let-bindings

[michaelpj]

This makes a few generated let-bindings strict. This has two effects:

1. We don't recompute the value if we use it multiple times in the body.
2. We are less likely to move the binding around or inline it because we can't tell if it's pure.

The latter is mildly worse, the former is moderately better. I wrote a budget test and it gets better after this commit, so it seems worth it.

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