[LoopUnroll] Avoid UB when converting from WeakVH to `Value *`

Calling `operator*` on a WeakVH with a null value yields a null
reference, which is UB. Avoid this by implicitly converting the WeakVH
to a `Value *` rather than dereferencing and then taking the address
for the type conversion.

Differential Revision: https://reviews.llvm.org/D73280