Add simplify_type<const WeakVH>; simplify IndVarSimplify

r240214 fixed some UB in IndVarSimplify, and it needed a temporary
`WeakVH` to do it.  Add `simplify_type<const WeakVH>` so that this
temporary isn't necessary.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240599 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed