[Transforms] Remove unused forward declaration LoopInfo

The last use was removed by:

  commit 1b12320
  Author: Alexandros Lamprineas <[email protected]>
  Date:   Thu May 11 13:44:40 2023 +0100

Author: kazutakahirata

llvm/include/llvm/Transforms/Utils/SCCPSolver.h

@@ -32,7 +32,6 @@ class Function;
 class GlobalVariable;
 class Instruction;
 class LLVMContext;
-class LoopInfo;
 class PostDominatorTree;
 class StructType;
 class TargetLibraryInfo;