NFC. Refactor affine fusion code for readability

Replace a couple of check instances with llvm::any_of (clang-tidy
warnings).  Factor out "canCreatePrivateMemRef" and
"performFusionsIntoDest" into separate methods to reduce the
length/indent of the containing methods. Add doc comments and debug messages.

Mark some of the methods that should have been const const.

NFC.

Reviewed By: vinayaka-polymage

Differential Revision: https://reviews.llvm.org/D142076