[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.

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

Subscribers: llvm-commits

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

llvm-svn: 328980