[WebAssembly] Use SetVector instead of SmallPtrSet in FixBrTableDefaults (#84418)

This pass inserts all the MBBs into a set and then iterates over them.
But when
the number of elements gets large enough, SmallPtrSet expands into a
hash table
which wouldn't have a deterministic iteration order since the elements
are
pointers. This results in nondeterministic jump table layouts.
Use SetVector instead for a deterministic iteration order.