[InstCombine] clean up wrap propagation for reassociated ops; NFCI

Always true/false checks were flagged by static analysis;
https://bugs.llvm.org/show_bug.cgi?id=43143

I have not confirmed the logic difference in propagating nsw vs. nuw,
but presumably we would have noticed a bug by now if that was wrong.

llvm-svn: 370248