`CeilDivUIOp` seemed to have been added by mistake to the list of
dynamically
illegal operations in `arith-unsigned-when-equivalent`. The only illegal
operations
should be the signed operations that can be converted to their unsigned
counterpart.
The pass was not checking for uninitialized states due to dead code.
This patch also makes LLVMFuncOp correctly return a null body when it is
external.
Fixes#58807
Depends on D139388
Reviewed By: rriddle
Differential Revision: https://reviews.llvm.org/D139389