[llvm] [MISched] Unify the way to specify scheduling direction (PR #119518)

[Pengcheng Wang via llvm-commits]

================
@@ -99,8 +99,16 @@
 
 namespace llvm {
 
-extern cl::opt<bool> ForceTopDown;
-extern cl::opt<bool> ForceBottomUp;
+namespace MISched {
+enum Direction {
+  Unspecified,
----------------
wangpc-pp wrote:

My point is that there will be a lot of `getNumOccurrences()`, which makes the conditions complicated. For example:
```
static ScheduleDAGInstrs *createInstructionShuffler(MachineSchedContext *C) {
  bool Alternate =
      PreRADirection != MISched::TopDown && PreRADirection != MISched::BottomUp;
  bool TopDown = PreRADirection != MISched::BottomUp;
  return new ScheduleDAGMILive(
      C, std::make_unique<InstructionShuffler>(Alternate, TopDown));
}
```
will be:
```
static ScheduleDAGInstrs *createInstructionShuffler(MachineSchedContext *C) {
  bool Alternate = PreRADirection.getNumOccurrences() > 0 &&
      PreRADirection != MISched::TopDown && PreRADirection != MISched::BottomUp;
  bool TopDown = PreRADirection.getNumOccurrences() > 0 && PreRADirection != MISched::BottomUp;
  return new ScheduleDAGMILive(
      C, std::make_unique<InstructionShuffler>(Alternate, TopDown));
}
```

https://github.com/llvm/llvm-project/pull/119518