Ensure memory access mappings are defined for full domain

We now verify that memory access functions imported via JSON are indeed defined
for the full iteration domain. Before this change we accidentally imported
memory mappings such as i -> i / 127, which only defined a mapped for values of
i that are evenly divisible by 127, but which did not define any mapping for the
remaining values, with the result that isl just generated an access expression
that had undefined behavior for all the unmapped values.

In the incorrect test cases, we now either use floor(i/127) or we use p/127 and
provide the information that p is indeed a multiple of 127.

llvm-svn: 239024