isl_ast_build_ast_from_schedule: avoid introducing iterator in hoisted condition

isl_ast_build_ast_from_schedule: avoid introducing iterator in hoisted condition

ast_graft_list_fuse was being passed the build of the inner level from
create_node_scaled, which could result in the current loop iterator
getting introduced in the hoisted condition.
Since we need the inner build for simplifying the expressions generated
from the conditions that are not hoisted out, we now pass two builds
to ast_graft_list_fuse.

Reported-by: Tobias Grosser <tobias@grosser.es>
Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>