[Reassociate] allow or->add with shl operands

As discussed in:
https://llvm.org/PR49055

We invert instcombine's add->or transform here
because it makes it easier to identify factorization
transforms like the mul in the motivating test.

This extends the logic added with:
https://reviews.llvm.org/rG70472f3
https://reviews.llvm.org/rG93f3d7f

(I intentionally kept the formatting fix in this patch
to provide more context about the calling logic.)