[llvm] Forward-declare ICFLoopSafetyInfo (NFC)

LoopUtils.h needs ICFLoopSafetyInfo but relies on a forward
declaration of ICFLoopSafetyInfo in IVDescriptors.h.  This patch adds
a forward declaration right in LoopUtils.h.

While we are at it, this patch removes the one in IVDescriptors.h,
where it is unnecessary.

Author: kazutakahirata

llvm/include/llvm/Analysis/IVDescriptors.h

@@ -32,7 +32,6 @@ class PredicatedScalarEvolution;
 class ScalarEvolution;
 class SCEV;
 class DominatorTree;
-class ICFLoopSafetyInfo;
 
 /// These are the kinds of recurrences that we support.
 enum class RecurKind {

llvm/include/llvm/Transforms/Utils/LoopUtils.h

@@ -27,6 +27,7 @@ class AliasSet;
 class AliasSetTracker;
 class BasicBlock;
 class BlockFrequencyInfo;
+class ICFLoopSafetyInfo;
 class IRBuilderBase;
 class Loop;
 class LoopInfo;