[SLP]Improve reordering of clustered reused scalars.

[SLP]Improve reordering of clustered reused scalars.

If the reused scalars are clustered, i.e. each part of the reused mask
contains all elements of the original scalars exactly once, we can
reorder those clusters to improve the whole ordering of of the clustered
vectors.

Differential Revision: https://reviews.llvm.org/D133524