MacroFusion: Fix macro fusion with ExitSU failing in top-down scheduling

When fusing instructions A and B, we must add all predecessors of B as
predecessors of A to avoid instructions getting scheduling in between.

There is a special case involving ExitSU: Every other node must be
scheduled before it by design and we don't need to make this explicit in
the graph, however when fusing with a different node we need to schedule
every othere node before the fused node too and we need to make this
explicit now: This patch adds a dependency from the fused node to all
roots in the graph.

Differential Revision: https://reviews.llvm.org/D49830

llvm-svn: 338046