[Scalarizer] InsertElement handling w/ constant insert index

Summary: As it can be clearly seen from the diff, this results in nicer IR.

Reviewers: jdoerfert, arsenm, bjope, cameron.mcinally

Reviewed By: jdoerfert

Subscribers: arphaman, wdng, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83102