[llvm] [llvm][CodeGen] avoid repeated interval calculation in window scheduler (PR #132352)

[Yuta Mukai via llvm-commits]

ytmukai wrote:

I added the following code to test the behavior when there is no interval. 

```diff
 void SwingSchedulerDAG::registerPressureFilter(NodeSetType &NodeSets) {
+  for (unsigned Reg = 0; Reg < MRI.getNumVirtRegs(); ++Reg) {
+    Register VReg = Register::index2VirtReg(Reg);
+    if (LIS.hasInterval(VReg))
+      LIS.removeInterval(VReg);
+  }
   for (auto &NS : NodeSets) {
```

As a result, there was no change in the nodes where `ExceedPressure` is set. When dumping LIS after `registerPressureFilter()`, only the registers defined in the target block were recalculated, so it seems there is no influence from other loops.

https://github.com/llvm/llvm-project/pull/132352