[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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@279626
91177308-0d34-0410-b5e6-
96231b3b80d8