inference: fix `[modifyfield!|replacefield!]_tfunc`s (#56310)
Currently the following code snippet results in an internal error:
```julia
julia> func(x) = @atomic :monotonic x[].count += 1;
julia> let;Base.Experimental.@force_compile
x = Ref(nothing)
func(x)
end
Internal error: during type inference of
...
```
This issue is caused by the incorrect use of `_fieldtype_tfunc(𝕃, o, f)`
within `modifyfield!_tfunc`, specifically because `o` should be
`widenconst`ed, but it isn’t. By using `_fieldtype_tfunc` correctly, we
can avoid the error through error-catching in `abstract_modifyop!`. This
commit also includes a similar fix for `replacefield!_tfunc` as well.
