[IR] Fix a memory leak if Function::dropAllReferences() is followed by setHungoffOperand

This patch fixes a memory leak if Function::dropAllReferences() is followed by setHungoffOperand (e.g. setPersonality)
If NumUserOperands changes from 3 to 0 before calling allocHungoffUselist() to allocate memory,
the memory leaks which are allocated when NumUserOperands is changed from 0 to 3.
e.g.
```
llvm::Function* func = ...;
func->setPersonalityFn(foo);  // (1). call allocHungoffUselist() to allocate memory for uses
func->deleteBody();  // (2). call dropAllReferences(), and it changes NumUserOperands from 3 to 0
// (3). at this point, NumUserOperands is 0, the next line will allocate memory by allocHungoffUselist()
func->setPersonalityFn(bar);  // (4). call allocHungoffUselist(), so memory allocated in (1) leaks.
```

Reviewed By: dexonsmith, MaskRay

Differential Revision: https://reviews.llvm.org/D156618