[ScheduleOptimizer] Allow tiling after fusion

In ScheduleOptimizer::isTileableBand(), allow the case in which
the band node's child is an isl_schedule_sequence_node and its
grandchildren isl_schedule_leaf_nodes. This case can arise when
two or more statements are fused by the isl scheduler.

The tile_after_fusion.ll test has two statements in separate
loop nests and checks whether they are tiled after being fused
when polly-opt-fusion equals "max".

Reviewers: grosser

Subscribers: gareevroman, pollydev

Tags: #polly

Contributed-by: Theodoros Theodoridis <theodort@student.ethz.ch>

Differential Revision: https://reviews.llvm.org/D30815

llvm-svn: 297587