[IPSCCP] Delete two forward declarations

[MaskRay]

Summary

Use forward declaration as the reviewer is in favor of #include and delete a redundant declaration of Function.

[fhahn]

Thanks for adding the include, I just saw the commit after I submitted the forward declaration.

If the forward declaration is sufficient (unfortunately the compiler I was using was clever enough to not require it...), I think it would be slightly better to remove the include. Both approaches LGTM, thank you very much!

[MaskRay]

Delete include

include/llvm/Transforms/Scalar/SCCP.h

 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/PassManager.h"
 #include "llvm/Transforms/Utils/PredicateInfo.h"
 
 namespace llvm {
 
-class Function;
-class PostDominatorTree;
-
 /// This pass performs function-level constant propagation and merging.
 class SCCPPass : public PassInfoMixin<SCCPPass> {
 public:
   PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
 };
 
 /// Helper struct for bundling up the analysis results per function for IPSCCP.
 struct AnalysisResultsForFn {