Optimise substr assignment in void context

In void context we can optimise

    substr($foo, $bar, $baz) = $replacement;

to something like

    substr($foo, $bar, $baz, $replacement);

except that the execution order must be preserved.  So what we actu-
ally do is

    substr($replacement, $foo, $bar, $baz);

with a flag to indicate that the replacement comes first.  This means
we can also optimise assignment to two-argument substr the same way.

Although optimisations are not supposed to change behaviour,
this one does.

• It stops substr assignment from calling get-magic twice, which means
  the optimisation makes things less buggy than usual.
• It causes the uninitialized warning (for an undefined first argu-
  ment) to mention the substr operator, as it did before the previous
  commit, rather than the assignment operator.  I think that sort of
  detail is minor enough.

I had to make the warning about clobbering references apply whenever
substr does a replacement, and not only when used as an lvalue.  So
four-argument substr now emits that warning.  I would consider that a
bug fix, too.

Also, if the numeric arguments to four-argument substr and the
replacement string are undefined, the order of the uninitialized warn-
ings is slightly different, but is consistent regardless of whether
the optimisation is in effect.

I believe this will make 95% of substr assignments run faster.  So
there is less incentive to use what I consider the less readable form
(the four-argument form, which is not self-documenting).

Since I like naïve benchmarks, here are Before and After:

$ time ./miniperl -le 'do{$x="hello"; substr ($x,0,0) = 34;0}for 1..1000000'

real	0m2.391s
user	0m2.381s
sys	0m0.005s
$ time ./miniperl -le 'do{$x="hello"; substr ($x,0,0) = 34;0}for 1..1000000'

real	0m0.936s
user	0m0.927s
sys	0m0.005s

1 files changed, 4 insertions, 0 deletions

diff --git a/op.h b/op.h
index 958529e306..d61198f28c 100644
--- a/op.h
+++ b/op.h
@@ -229,6 +229,10 @@ Deprecated.  Use C<GIMME_V> instead.
   /* OP_RV2[AGH]V, OP_PAD[AH]V, OP_[AH]ELEM, OP_[AH]SLICE OP_AV2ARYLEN,
      OP_R?KEYS, OP_SUBSTR, OP_POS, OP_VEC */
 #define OPpMAYBE_LVSUB		8	/* We might be an lvalue to return */
+
+  /* OP_SUBSTR only */
+#define OPpSUBSTR_REPL_FIRST	16	/* 1st arg is replacement string */
+
   /* OP_PADSV only */
 #define OPpPAD_STATE		16	/* is a "state" pad */
   /* for OP_RV2?V, lower bits carry hints (currently only HINT_STRICT_REFS) */