compiler: unify singleton_type and argtype_to_function (#42342)

They are supposed to do the same thing.
Previously `singleton_type` didn't handle the case where
`isconstType(ft) -> ft.parameters[1]`, but it can just include that.

base/compiler/abstractinterpretation.jl

@@ -1235,7 +1235,7 @@ function abstract_invoke(interp::AbstractInterpreter, argtypes::Vector{Any}, sv:
     #     t, a = ti.parameters[i], argtypes′[i]
     #     argtypes′[i] = t ⊑ a ? t : a
     # end
-    const_result = abstract_call_method_with_const_args(interp, result, argtype_to_function(ft′), argtypes′, match, sv, false)
+    const_result = abstract_call_method_with_const_args(interp, result, singleton_type(ft′), argtypes′, match, sv, false)
     if const_result !== nothing
         const_rt, const_result = const_result
         if const_rt !== rt && const_rt ⊑ rt
@@ -1379,7 +1379,7 @@ function abstract_call(interp::AbstractInterpreter, fargs::Union{Nothing,Vector{
                        sv::InferenceState, max_methods::Int = InferenceParams(interp).MAX_METHODS)
     #print("call ", e.args[1], argtypes, "\n\n")
     ft = argtypes[1]
-    f = argtype_to_function(ft)
+    f = singleton_type(ft)
     if isa(ft, PartialOpaque)
         return abstract_call_opaque_closure(interp, ft, argtypes[2:end], sv)
     elseif (uft = unwrap_unionall(ft); isa(uft, DataType) && uft.name === typename(Core.OpaqueClosure))
@@ -1396,18 +1396,6 @@ function abstract_call(interp::AbstractInterpreter, fargs::Union{Nothing,Vector{
     return abstract_call_known(interp, f, fargs, argtypes, sv, max_methods)
 end
 
-function argtype_to_function(@nospecialize(ft))
-    if isa(ft, Const)
-        return ft.val
-    elseif isconstType(ft)
-        return ft.parameters[1]
-    elseif isa(ft, DataType) && isdefined(ft, :instance)
-        return ft.instance
-    else
-        return nothing
-    end
-end
-
 function sp_type_rewrap(@nospecialize(T), linfo::MethodInstance, isreturn::Bool)
     isref = false
     if T === Bottom

base/compiler/ssair/inlining.jl

@@ -717,15 +717,6 @@ function rewrite_invoke_exprargs!(argexprs::Vector{Any})
     return argexprs
 end
 
-function singleton_type(@nospecialize(ft))
-    if isa(ft, Const)
-        return ft.val
-    elseif ft isa DataType && isdefined(ft, :instance)
-        return ft.instance
-    end
-    return nothing
-end
-
 function compileable_specialization(et::Union{EdgeTracker, Nothing}, match::MethodMatch)
     mi = specialize_method(match; compilesig=true)
     mi !== nothing && et !== nothing && push!(et, mi::MethodInstance)

base/compiler/utilities.jl

@@ -222,6 +222,10 @@ function method_for_inference_heuristics(method::Method, @nospecialize(sig), spa
     return nothing
 end
 
+#########
+# types #
+#########
+
 argextype(@nospecialize(x), state) = argextype(x, state.src, state.sptypes, state.slottypes)
 
 const empty_slottypes = Any[]
@@ -259,6 +263,17 @@ function argextype(@nospecialize(x), src, sptypes::Vector{Any}, slottypes::Vecto
     end
 end
 
+function singleton_type(@nospecialize(ft))
+    if isa(ft, Const)
+        return ft.val
+    elseif isconstType(ft)
+        return ft.parameters[1]
+    elseif ft isa DataType && isdefined(ft, :instance)
+        return ft.instance
+    end
+    return nothing
+end
+
 ###################
 # SSAValues/Slots #
 ###################