isl_union_*: use tuple hash in hash table

isl_union_*: use tuple hash in hash table

This makes the ordering of the base expressions within a union expression
independent of the parameters.

Since all base expressions in the union expression have their parameters
aligned with the union, there is no need to check
for equal parameters.  In any case, any union expression
can only contain a single base expression in a given tuple space
even if this alignment were not performed.

An upcoming commit will change the comparison of entries
with the same hash value to ignore parameters as well.
In fact, union expressions that contain at most a single
base expression per domain space already ignore parameters there.
Together, these two changes ensure that look-ups can be performed
without first aligning parameters.

Signed-off-by: Sven Verdoolaege <sven.verdoolaege@gmail.com>