[LoopDeletion] Fix deletion with unusual predecessor terminator (PR56266)

LoopSimplify only requires that the loop predecessor has a single
successor and is safe to hoist into -- it doesn't necessarily have
to be an unconditional BranchInst.

Adjust LoopDeletion to assert conditions closer to what it actually
needs for correctness, namely a single successor and a
side-effect-free terminator (as the terminator is getting dropped).

Fixes https://github.com/llvm/llvm-project/issues/56266.