[LoopInterchange] Fix handling of LCSSA nodes defined in headers and latches.

The code to preserve LCSSA PHIs currently only properly supports
reduction PHIs and PHIs for values defined outside the latches.

This patch improves the LCSSA PHI handling to cover PHIs for values
defined in the latches.

Fixes PR41725.

Reviewers: efriedma, mcrosier, davide, jdoerfert

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D61576

llvm-svn: 361743