[Transforms] Remove unused forward declaration AAResults

The last use was removed by:

  commit 41b45ce
  Author: Florian Hahn <[email protected]>
  Date:   Mon Dec 19 20:37:47 2022 +0000

Author: kazutakahirata

llvm/include/llvm/Transforms/Vectorize/LoopVectorizationLegality.h

@@ -32,7 +32,6 @@
 #include "llvm/Transforms/Utils/LoopUtils.h"
 
 namespace llvm {
-class AAResults;
 class AssumptionCache;
 class BasicBlock;
 class BlockFrequencyInfo;