v0.0.0-6045-gd0c9bbd4d

Make codegen implement shra as a function.

This allows us to bind the intermediate result `$signed(to_shift) >>> amount` to a reg with a defined width, which avoids the `$signed()` having a self-determined width, which is a lint finding.

Fixes google/xls#1642.

PiperOrigin-RevId: 682526771