Fix EXTRACT_LAST_REDUCTION segfault

Fix EXTRACT_LAST_REDUCTION segfault

This code:

      /* Make sure we don't accidentally use the old condition.  */
      cond_expr = NULL_TREE;

was misplaced, since it triggered even when we needed to force the
original unmodified cond_expr into a mask temporary and then invert it.

2019-12-31  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-stmts.c (vectorizable_condition): Only nullify cond_expr
	if we've created a new condition.  Don't nullify it if we've decided
	to keep it and then invert the result.

gcc/testsuite/
	* gcc.dg/vect/vect-cond-reduc-6.c: New test.

From-SVN: r279804