[DeLICM] Always normalize domain. NFC.

Some isl functions can simplify their __isl_keep arguments. The
argument object after the call uses different contraints to represent
the same set. Different contraints can result in different outputs
when printed to a string.

In assert builds additional isl functions are called (in assert() or
mentioned, these can change the internal representation of its read-only
arguments such that printed strings are different in debug and non-debug
builds.

What happened here is that a call to isl_set_is_equal inside an assert
in getScatterFor normalizes one of its arguments such that one redundant
constraint is removed. The redundant constraint therefore does not appear
in the string representing the domain, which FileCheck notices as a
regression test failure compared to a build with assertions disabled.

This fix removes the redundant contraints the domain from the start such
that the redundant contraint is removed in assert and non-assert builds.
Isl adds a flag to such sets such that the removal of redundancies is
not done multiple times (here: by isl_set_is_equal).

Thanks to Tobias Grosser for reporting and hinting to the cause.

llvm-svn: 302711