inference: fix [modifyfield!|replacefield!]_tfuncs (#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.

base/compiler/tfuncs.jl

@@ -1347,13 +1347,15 @@ end
     return getfield_tfunc(𝕃, o, f)
 end
 @nospecs function modifyfield!_tfunc(𝕃::AbstractLattice, o, f, op, v, order=Symbol)
-    T = _fieldtype_tfunc(𝕃, o, f, isconcretetype(o))
+    o′ = widenconst(o)
+    T = _fieldtype_tfunc(𝕃, o′, f, isconcretetype(o′))
     T === Bottom && return Bottom
     PT = Const(Pair)
     return instanceof_tfunc(apply_type_tfunc(𝕃, PT, T, T), true)[1]
 end
 @nospecs function replacefield!_tfunc(𝕃::AbstractLattice, o, f, x, v, success_order=Symbol, failure_order=Symbol)
-    T = _fieldtype_tfunc(𝕃, o, f, isconcretetype(o))
+    o′ = widenconst(o)
+    T = _fieldtype_tfunc(𝕃, o′, f, isconcretetype(o′))
     T === Bottom && return Bottom
     PT = Const(ccall(:jl_apply_cmpswap_type, Any, (Any,), T) where T)
     return instanceof_tfunc(apply_type_tfunc(𝕃, PT, T), true)[1]

test/compiler/inference.jl

@@ -6063,3 +6063,18 @@ end === Union{}
 @test Base.infer_return_type() do
     TypeVar(:Issue56248, Any, 1)
 end === Union{}
+
+@test Base.infer_return_type((Nothing,)) do x
+    @atomic x.count += 1
+end == Union{}
+@test Base.infer_return_type((Nothing,)) do x
+    @atomicreplace x.count 0 => 1
+end == Union{}
+mutable struct AtomicModifySafety
+    @atomic count::Int
+end
+let src = code_typed((Union{Nothing,AtomicModifySafety},)) do x
+        @atomic x.count += 1
+    end |> only |> first
+    @test any(@nospecialize(x)->Meta.isexpr(x, :invoke_modify), src.code)
+end