[llvm] Reland "CodeGen][NewPM] Port MachineScheduler to NPM. (#125703)" (PR #126684)

[Matt Arsenault via llvm-commits]

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@@ -98,6 +98,10 @@
 #include <vector>
 
 namespace llvm {
+namespace impl_detail {
+class MachineSchedulerImpl;
+class PostMachineSchedulerImpl;
+} // namespace impl_detail
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arsenm wrote:

I don't think these should need to be exposed in the header 

https://github.com/llvm/llvm-project/pull/126684