`unsafe_load` does not taint `:noub` effect

```julia
do_ub() = unsafe_load(Ptr{Int}(UInt(0)))
```

This function executes undefined behavior, but the compiler has no idea somehow:
```julia
julia> Base.infer_effects(do_ub, ())
(!c,+e,+n,+t,+s,+m,+u,+o,+r)
julia> do_ub()
Warning: detected a stack overflow; program state may be corrupted, so further execution might be unreliable.
ERROR: StackOverflowError:
```