[Scalarizer] Do not insert instructions between PHI nodes and debug intrinsics.

The scalarizer pass seems to be inserting instructions in-between PHI nodes or debug intrinsics that end up staying at the end of the pass, resulting in malformed IR and violating assumptions.

This patch adds a check to make sure the `extractelement` instructions that it adds are correctly placed after all PHI nodes and debug intrinsics.

Patch by vettoreldaniele.

Reviewed By: bjope

Differential Revision: https://reviews.llvm.org/D112472