IndVarSimplify: Avoid UB from binding a reference to a null pointer

IndVarSimplify: Avoid UB from binding a reference to a null pointer

Calling operator* on a WeakVH whose Value is null hits undefined
behaviour, since we bind the value to a reference. Instead, go through
`operator Value*` so that we work with the pointer itself.

Found by ubsan.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240214 91177308-0d34-0410-b5e6-96231b3b80d8