A few weeks ago I committed this patch to fix some problems I saw in 
nios2-elf testing:

https://gcc.gnu.org/ml/gcc-patches/2020-01/msg01385.html

Since then there have been some further new C++ constexpr testcases 
added that have the same problem with assuming 
-fdelete-null-pointer-checks.  I've committed the attached patch as an 
obvious followup to the last one.

-Sandra