Fix reduce to preserve its input across init backtracking (#3633)

`reduce f as $x (init; update)` evaluated `f` against a stack slot
that forkpoints created while evaluating `init` restore. Both the
`DUPN` opening the reduction loop and the `LOADVN` retrieving the
result pop that slot destructively, nulling it out, so an `init`
that backtracks (e.g. `0, 0`) made `f` see `null` and fail with
"Cannot iterate over null".

Duplicate the input before the `FORK` instead of inside the loop,
and drop the leftover copy with `POP` once the loop is exhausted.
The slot left live at the `FORK` is then a block pushed after
`init` completed, which no earlier forkpoint can reach, so nulling
it is harmless. This is the same approach `gen_foreach` has used
since c70ae1a.

`DUPN` had no other user, so remove the opcode.

src/compile.c

@@ -832,8 +832,7 @@ static block bind_alternation_matchers(block matchers, block body) {
 
 block gen_reduce(block source, block matcher, block init, block body) {
   block res_var = gen_op_var_fresh(STOREV, "reduce");
-  block loop = BLOCK(gen_op_simple(DUPN),
-                     source,
+  block loop = BLOCK(source,
                      bind_alternation_matchers(matcher,
                                   BLOCK(gen_op_bound(LOADVN, res_var),
                                         body,
@@ -842,8 +841,10 @@ block gen_reduce(block source, block matcher, block init, block body) {
   return BLOCK(gen_op_simple(DUP),
                init,
                res_var,
+               gen_op_simple(DUP),
                gen_op_target(FORK, loop),
                loop,
+               gen_op_simple(POP),
                gen_op_bound(LOADVN, res_var));
 }