[InstCombine] don't assume 'inbounds' for bitcast deref or null pointer in...

[InstCombine] don't assume 'inbounds' for bitcast deref or null pointer in non-default address space

Follow-up to D68244 to account for a corner case discussed in:
https://bugs.llvm.org/show_bug.cgi?id=43501

Add one more restriction: if the pointer is deref-or-null and in a non-default
(non-zero) address space, we can't assume inbounds.

Differential Revision: https://reviews.llvm.org/D68706

llvm-svn: 374728