[SLP] Fix PR35777: Incorrect handling of aggregate values.

Summary:
Fixes the bug with incorrect handling of InsertValue|InsertElement
instrucions in SLP vectorizer. Currently, we may use incorrect
ExtractElement instructions as the operands of the original
InsertValue|InsertElement instructions.

Reviewers: mkuper, hfinkel, RKSimon, spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D41767

llvm-svn: 321994