Fix bad transformation in division simplification

[meheff]

The arithmetic simplification pass checks whether y << k can overflow before rewriting:

   x / (y << k)  →  (x / y) >> k

But it converted the unsigned maximum shift using signed ToInt64() which is incorrect as the semantics require interpretation of the value as an unsigned number.

[google-cla]

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[meheff]

Looks like the cla error is due to an codex push which added a local email address meheff@<somerandommachine.net>. Feel free to just clone this and submit the fix internally.

[ericastor]

@meheff - we definitely shouldn't just clone & submit internally with a CLA violation flagged. Any chance you can get it fixed on your end?

[ericastor]

I realized I have not looked at the actual content of this PR. As such, I'm taking this as a bug report instead & will fix it separately.

[meheff]

The CLA issue is that codex added meheff@mylocalmachinename.net as a coauthor. My own CLA is still up to date. I reopened this as a new PR #4949 without this problem.