tree-ifcvt: Add zero maskload else value.

When predicating a load we implicitly assume that the else value is
zero.  This matters in case the loaded value is padded (like e.g.
a Bool) and we must ensure that the padding bytes are zero on targets
that don't implicitly zero inactive elements.

A former version of this patch still had this handling in ifcvt but
the latest version defers it to the vectorizer.

gcc/ChangeLog:

	* tree-if-conv.cc (predicate_load_or_store): Add zero else
	operand and comment.