[LoopInterchange] Fix interchanging contents of preheader BBs

Summary:
Previously LCSSA was getting broken by placing instructions into the
(newly) inner *header* instead of the *pre*header.

Fixes PR43474

Reviewers: fhahn

Reviewed By: fhahn

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75943