This patch adds the `cir.ffs` operation which corresponds to the
`__builtin_ffs` family of builtin functions.
This operation was not included in the previous PRs because the call to
`__builtin_ffs` would be transformed into a library call to `ffs`. At
the time of authoring this patch, this behavior has been changed and now
we can properly lower calls to `__builtin_ffs` to `cir.ffs`.
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