[LSR] Replace casts with an equivalent std::as_const (NFC) (#138980)

The casts / `std::as_const` are used here to select `const` overload of
`begin()`/`end()` so that the type of the returned iterator matches the
type of `J`, which is `const_iterator`.

llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp

@@ -3946,10 +3946,8 @@ void LSRInstance::GenerateReassociationsImpl(LSRUse &LU, unsigned LUIdx,
       continue;
 
     // Collect all operands except *J.
-    SmallVector<const SCEV *, 8> InnerAddOps(
-        ((const SmallVector<const SCEV *, 8> &)AddOps).begin(), J);
-    InnerAddOps.append(std::next(J),
-                       ((const SmallVector<const SCEV *, 8> &)AddOps).end());
+    SmallVector<const SCEV *, 8> InnerAddOps(std::as_const(AddOps).begin(), J);
+    InnerAddOps.append(std::next(J), std::as_const(AddOps).end());
 
     // Don't leave just a constant behind in a register if the constant could
     // be folded into an immediate field.