[SLP]Fix an assertion for the size of user nodes.

For the nodes with reused scalars the user may be not only of the size
of the final shuffle but also of the size of the scalars themselves,
need to check for this. It is safe to just modify the check here, since
the order of the scalars themselves is preserved, only indeces of the
reused scalars are changed. So, the users with the same size as the
number of scalars in the node, will not be affected, they still will get
the operands in the required order.

Reported by @mstorsjo in D105020.

Differential Revision: https://reviews.llvm.org/D107080