[InstCombine] add assert and explanatory comment for fold removed in r279568; NFC

I deleted a fold from InstCombine at:
https://reviews.llvm.org/rL279568

because it (like any InstCombine to a constant?) should always happen in InstSimplify,
however, it's not obvious what the assumptions are in the remaining code.

Add a comment and assert to make it clearer.

Differential Revision: https://reviews.llvm.org/D23819

llvm-svn: 279626