Avoid vivifying stuff when looking up barewords

Till now, when a bareword was looked up to see whether it was a sub-
routine, an rv2cv op was created (to allow PL_check hooks to override
the process), which was then asked for its GV.

Afterwards, the GV was downgraded back to nothing if possible.

So a lot of the time a GV was autovivified and then discarded.  This
has been the case since f74617600 (5.12).

If we know there is a good chance that the rv2cv op is about to be
deleted, we can avoid that by passing a flag to the new op.

Also f74617600 actually changed the behaviour by vivifying stashes
that used not be vivified:

sub foo { print shift, "\n" }
SUPER::foo bar if 0;
foo SUPER;

Output in 5.10:

    SUPER

Output as of this commit:

    SUPER

Output in 5.12 to 5.21.3:

    Can't locate object method "foo" via package "SUPER" at - line 3.

1 files changed, 10 insertions, 1 deletions

diff --git a/t/op/stash.t b/t/op/stash.t
index 598811487d..4c846b7ec4 100644
--- a/t/op/stash.t
+++ b/t/op/stash.t
@@ -7,7 +7,7 @@ BEGIN {
 
 BEGIN { require "./test.pl"; }
 
-plan( tests => 49 );
+plan( tests => 50 );
 
 # Used to segfault (bug #15479)
 fresh_perl_like(
@@ -318,3 +318,12 @@ ok eval '
      sub foo{};
      1
   ', 'no crashing or errors when clobbering the current package';
+
+# Bareword lookup should not vivify stashes
+is runperl(
+    prog =>
+      'sub foo { print shift, qq-\n- } SUPER::foo bar if 0; foo SUPER',
+    stderr => 1,
+   ),
+   "SUPER\n",
+   'bareword lookup does not vivify stashes';