Changes

[MacroFusion] Limit the max fused number as 2 to reduce the dependency
This is the example:
int foo(int a, int b, int c, int d) {
 return a + b + c + d;
}
And this is the Dependency Graph:
+------+       +------+       +------+       +------+
|  A   |       |  B   |       |  C   |       |  D   |
+--+--++       +---+--+       +--+---+       +--+---+
  ^  ^            ^  ^          ^              ^
  |  |            |  |          |              |
  |  |            |  |New1      +--------------+
  |  |            |  |          |
  |  |            |  |       +--+---+
  |  |New2        |  +-------+ ADD1 |
  |  |            |          +--+---+
  |  |            |    Fuse     ^
  |  |            +-------------+
  |  +------------+
  |               |
  |   Fuse     +--+---+
  +----------->+ ADD2 |
  |            +------+
+--+---+
| ADD3 |
+------+
We need also create an artificial edge from ADD1 to A if 
https://reviews.llvm.org/D69998 is landed. That will force the Node A
scheduled before the ADD1 and ADD2. But in fact, it is ok to schedule
the Node A in-between ADD3 and ADD2, as ADD3 and ADD2 are NOT a fusion
pair because ADD2 has been matched to ADD1. We are creating these
unnecessary dependency edges that override the heuristics.
Differential Revision: https://reviews.llvm.org/D70066