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.
(cherry picked from commit e416b09711039b65a37b6c83977fb04c2c0889ea)
Marc Auberer
8d6df3124d