[llvm] IR: Remove null UseList checks in hasNUses methods (PR #165929)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> @arsenm it wasn't relying on uselists of a constant data. It was relying on `hasNUses` call on `Value` (just instruction operands). In this case it was trivial to add a dyn_cast check but currently `hasNUses` feels footgun-y. All "unqualified" (i.e. on plain Value) calls are now suspect.
> 
> Also, `hasOneUse` still has the check, if you want to go all the way.

That's a pain. Probably should revert this then 

https://github.com/llvm/llvm-project/pull/165929