[Simplify] Improve scalability.

With a lot of reads and writes to the same array in a statement,
some isl sets that capture the state between access can become
complex such that isl takes more considerable time and memory
for operations on them.

The problems identified were:

- is_subset() takes considerable time with many disjoints in the
  arguments. We limit the number of disjoints to 4, any additional
  information is thrown away.

- subtract() can lead to many disjoints. We instead assume that any
  array element is possibly accessed, which removes all disjoints.

- subtract_domain() may lead to considerable processing, even if all
  elements are are to be removed. Instead, we remove determine and
  remove the affected spaces manually. No behaviour is changed.

llvm-svn: 309728