irinterp: Don't try to access mi.def if it's not a Method (#54003)

I don't believe this is reachable from the base compiler pipeline, since
we don't run irinterp on toplevel things, but I am seeing this in
downstream packages.

base/compiler/inferencestate.jl

@@ -783,9 +783,13 @@ mutable struct IRInterpretationState
         for i = 1:length(given_argtypes)
             given_argtypes[i] = widenslotwrapper(argtypes[i])
         end
-        given_argtypes = va_process_argtypes(optimizer_lattice(interp), given_argtypes, mi)
-        argtypes_refined = Bool[!⊑(optimizer_lattice(interp), ir.argtypes[i], given_argtypes[i])
-            for i = 1:length(given_argtypes)]
+        if isa(mi.def, Method)
+            given_argtypes = va_process_argtypes(optimizer_lattice(interp), given_argtypes, mi)
+            argtypes_refined = Bool[!⊑(optimizer_lattice(interp), ir.argtypes[i], given_argtypes[i])
+                for i = 1:length(given_argtypes)]
+        else
+            argtypes_refined = Bool[false for i = 1:length(given_argtypes)]
+        end
         empty!(ir.argtypes)
         append!(ir.argtypes, given_argtypes)
         tpdum = TwoPhaseDefUseMap(length(ir.stmts))