Revert "Remove certificates on safe statements."
Revert "Remove certificates on safe statements."

This reverts commit 98db1fdb53ff6b76aa85dc84547da4cb7f8fd751.

Turns out this broke user-provided assertions.

diku-dk/futhark,diku-dk/futhark,diku-dk/futhark,HIPERFIT/futhark,HIPERFIT/futhark,diku-dk/futhark,HIPERFIT/futhark,diku-dk/futhark