Use new constant function instead of deleted makeScalar function

[mlevesquedion]

This seems to be from #1987 (see https://github.com/openxla/stablehlo/actions/runs/7808382192/job/21298492956).

Not sure how presubmit failed to catch this. I think what happened is the following:

1. Rename function to closely match spec #1987 was uploaded and passed presubmit.
2. Add interpreter for CollectiveBroadcastOp #1983 was submitted, adding a new use of makeScalar.
3. Rename function to closely match spec #1987 was submitted, deleting makeScalar and causing tests to fail at HEAD.

What I don't understand is why this sequence of operations is possible in the first place. I feel like when #1983 was submitted, the earlier passing presubmit on #1987 should have been invalidated, blocking submit. I guess presubmits only re-run if you add a commit? So in this case since there was no new commit and no conflict, no new commit needed to be pushed and the presubmits were not run again.

In the grand scheme of things, I think this is fairly minor, but we may want to investigate whether we can prevent this somehow by tweaking some configuration settings or something.

[ghpvnist]

Interesting edge case, I agree preventing this could be useful but likely to be rare (this is the first time encountering this as far as I remember). Thanks for fixing this!