[SLP]Do not reorder top nodes if they do not require reordering.

No need to reorder the top nodes, if they are not stores or
insertelement instructions and each node should be analized only
once, when the bottom-to-top analysis is performed.
We still endup with extractelements for the top node scalars and
the final shuffle just adds an extra cost and currently
crashes the compiler for PHI nodes.

Differential Revision: https://reviews.llvm.org/D116760