isl: "isl_schedule_get_map: handle trees with divergent filter node parameters"

Also un-revert (isl_pw_*_alloc: add missing check for compatible spaces, Wed Sep
6 12:18:04 2017 +0200).

This patch is a proposed fix to avoid asserts due to stricter space checking
within isl, which resulted in failures when converting a schedule tree to
a schedule map.

llvm-svn: 326073