[Scalarizer] ExtractElement handling w/ variable insert index (PR46524)

Summary:
Similar to D82961.

Reviewers: bjope, cameron.mcinally, arsenm, jdoerfert

Reviewed By: jdoerfert

Subscribers: arphaman, wdng, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82970