Allow constant propagation of mutables with const fields (#58371)

This make the ScopedValue code slightly better by folding away the
has_default and default lookups

Still needs a test

Author: gbaraldi

Compiler/src/abstractlattice.jl

@@ -227,9 +227,10 @@ that should be forwarded along with constant propagation.
 end
 @nospecializeinfer function is_const_prop_profitable_arg(𝕃::ConstsLattice, @nospecialize t)
     if isa(t, Const)
-        # don't consider mutable values useful constants
+        # don't consider mutable values useful constants unless they have const fields
         val = t.val
-        return isa(val, Symbol) || isa(val, Type) || isa(val, Method) || isa(val, CodeInstance) || !ismutable(val)
+        return isa(val, Symbol) || isa(val, Type) || isa(val, Method) || isa(val, CodeInstance) ||
+                    !ismutable(val) || (typeof(val).name.constfields != C_NULL)
     end
     isa(t, PartialTypeVar) && return false # this isn't forwardable
     return is_const_prop_profitable_arg(widenlattice(𝕃), t)

Compiler/test/irpasses.jl

@@ -2111,3 +2111,13 @@ let src = code_typed1(()) do
     end
     @test count(iscall((src, isdefined)), src.code) == 0
 end
+# We should successfully fold the default values of a ScopedValue
+const svalconstprop = ScopedValue(1)
+foosvalconstprop() = svalconstprop[]
+
+let src = code_typed1(foosvalconstprop, ())
+    function is_constfield_load(expr)
+        iscall((src, getfield))(expr) && expr.args[3] in (:(:has_default), :(:default))
+    end
+    @test count(is_constfield_load, src.code) == 0
+end