fix 'for reverse @array' bug on AIX

RT #133558

Due to what appears to be a compiler bug on AIX (or perhaps it's
undefined behaviour which happens to work on other platforms), this line
of code in pp_iter():

    inc = 1 - (PL_op->op_private & OPpITER_REVERSED);

was setting inc to 4294967295 rather than to the expected -1 (inc was a
64-bit signed long).

Fix it with a couple of judicious (IV) casts (which ought to be a NOOP).

pp_hot.c

@@ -3932,7 +3932,7 @@ PP(pp_iter)
     case CXt_LOOP_LIST: /* for (1,2,3) */
 
         assert(OPpITER_REVERSED == 2); /* so inc becomes -1 or 1 */
-        inc = 1 - (PL_op->op_private & OPpITER_REVERSED);
+        inc = (IV)1 - (IV)(PL_op->op_private & OPpITER_REVERSED);
         ix = (cx->blk_loop.state_u.stack.ix += inc);
         if (UNLIKELY(inc > 0
                         ? ix > cx->blk_oldsp
@@ -3947,7 +3947,7 @@ PP(pp_iter)
     case CXt_LOOP_ARY: /* for (@ary) */
 
         av = cx->blk_loop.state_u.ary.ary;
-        inc = 1 - (PL_op->op_private & OPpITER_REVERSED);
+        inc = (IV)1 - (IV)(PL_op->op_private & OPpITER_REVERSED);
         ix = (cx->blk_loop.state_u.ary.ix += inc);
         if (UNLIKELY(inc > 0
                         ? ix > AvFILL(av)

t/op/for.t

@@ -5,7 +5,7 @@ BEGIN {
     require "./test.pl";
 }
 
-plan(124);
+plan(126);
 
 # A lot of tests to check that reversed for works.
 
@@ -664,3 +664,17 @@ is(fscope(), 1, 'return via loop in sub');
     }
     is($foo, "outside", "RT #123994 array outside");
 }
+
+# RT #133558 'reverse' under AIX was causing loop to terminate
+# immediately, probably due to compiler bug
+
+{
+    my @a = qw(foo);
+    my @b;
+    push @b, $_ for (reverse @a);
+    is "@b", "foo", " RT #133558 reverse array";
+
+    @b = ();
+    push @b, $_ for (reverse 'bar');
+    is "@b", "bar", " RT #133558 reverse list";
+}