[DivRemPairs] make sure we have a valid CFG for hoisting division

This transform was added with e38b7e89
and as shown in:
https://llvm.org/PR51241
...it could crash without an extra check of the blocks.

There might be a more compact way to write this constraint,
but we can't just count the successors/predecessors without
affecting a test that includes a switch instruction.

(cherry picked from commit 5b83261c)