Protect statements hoisted out of SegMap.

Previously we could hoist out indexings and assertions that were strictly
speaking only valid if the segmap had a nonzero trip count.

Unfortunately this had some carry-on effects elsewhere. I'm a little dubious of
marking slicing as safe, even though *on its own* it cannot fail.

This is 0.26.3.

This is 0.26.2.

This is 0.26.1.

This is 0.25.37.

This is 0.25.36.

This is 0.25.35.

This is 0.25.34.

This is 0.25.33.

This is 0.25.32.