Iterate over registered impls in reverse order

Going through the registered implementations in the opposite order that
they were defined means that implementations that are defined later in a
file are used instead of the ones earlier in the file when they overlap.

mypyc/irbuild/function.py

@@ -779,7 +779,9 @@ def add_singledispatch_registered_impls(builder: IRBuilder) -> None:
     line = fitem.line
     current_func_decl = builder.mapper.func_to_decl[fitem]
     arg_info = get_args(builder, current_func_decl.sig.args, line)
-    for dispatch_type, impl in impls:
+    # Reverse the list of registered implementations so we use the implementations defined later
+    # if there are multiple overlapping implementations
+    for dispatch_type, impl in reversed(impls):
         func_decl = builder.mapper.func_to_decl[impl]
         call_impl, next_impl = BasicBlock(), BasicBlock()
         should_call_impl = check_if_isinstance(builder, arg_info.args[0], dispatch_type, line)