[InstCombine] don't assume 'inbounds' for bitcast pointer to GEP transform (PR43501)

https://bugs.llvm.org/show_bug.cgi?id=43501
We can't declare a GEP 'inbounds' in general. But we may salvage that information if
we have known dereferenceable bytes on the source pointer.

Differential Revision: https://reviews.llvm.org/D68244

llvm-svn: 373847