[SLP] Fix PR36481: vectorize reassociated instructions.

Summary:
If the load/extractelement/extractvalue instructions are not originally
consecutive, the SLP vectorizer is unable to vectorize them. Patch
allows reordering of such instructions.

Patch does not support reordering of the repeated instruction, this must
be handled in the separate patch.

Reviewers: RKSimon, spatel, hfinkel, mkuper, Ayal, ashahid

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D43776

llvm-svn: 329085