Fix PR 107734: valgrind errors with sbitmap in match.pd

sbitmap is a simple bitmap and the memory allocated is not cleared
on creation; you have to clear it or set it to all ones before using
it.  This is unlike bitmap which is a sparse bitmap and the entries are
cleared as created.
The code added in r13-4044-gdc95e1e9702f2f missed that.
This patch fixes that mistake.

Committed as obvious after a bootstrap and test on x86_64-linux-gnu.

gcc/ChangeLog:

	PR middle-end/107734
	* match.pd (perm + vector op pattern): Clear the sbitmap before
	use.

gcc/match.pd

@@ -8288,6 +8288,8 @@ and,
 	   if (sel.encoding ().encoded_full_vector_p ())
 	     {
 	       auto_sbitmap seen (nelts);
+	       bitmap_clear (seen);
+
 	       unsigned HOST_WIDE_INT count = 0, i;
 
 	       for (i = 0; i < nelts; i++)