[IR] Set Number of Hungoff Uses after allocation (#213304)
If allocation of the Use[] in allocHungoffUses fails (e.g. due to OOM),
it may happen that the destructor of User is called with NumUserOperands
> 0. Then `Use::zap` tries to destruct Uses that were not constructed in
the first place.
Example scenario for SwitchInst:
`SwitchInst::init()` is called, which means the constructor of the
parent class User went through successfully, which means the destructor
of User will be called eventually. If now `SwitchInst::init()` calls
first `setNumHungOffUseOperands(2)` and then `allocHungoffUses()` and
the allocation there fails, `~User()` is called and tries to destruct 2
Uses, which can lead to problems.
This patch consequently ensures that first the allocation happens and
then NumUserOperands is set accordingly, so that this problem is
prevented.
