Use a worklist to avoid a sneaky iterator invalidation.

The iterator could be invalidated when it's recursively deleting a whole bunch
of constant expressions in a constant initializer.

Note: This was only reproducible if `opt' was run on a `.bc' file. If `opt' was
run on a `.ll' file, it wouldn't crash. This is why the test first pushes the
`.ll' file through `llvm-as' before feeding it to `opt'.

PR15440

llvm-svn: 178531