[Scalarizer] ExtractElement handling w/ constant extract index

Summary:
It appears to be better IR-wise to aggressively scalarize it,
rather than relying on gathering it, and leaving it as-is.

Reviewers: jdoerfert, bjope, arsenm, cameron.mcinally

Reviewed By: jdoerfert

Subscribers: arphaman, wdng, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83101