Add Scalarization Patterns for AtenToDtypeOp, AtenNegOp, AtenRemainderTensorOp

[zjgarvey]

1. adds a lowering for aten.neg.int and aten.remainder.int to arith.
2. adds a scalarization pattern for aten.neg and aten.remainder.Tensor ops.
3. improves folding of aten.mul.int
4. adds a scalarization pattern for aten.to.dtype which relies on scalar cast ops and basic C++ casting between double and int64_t.
5. improves rank-0 case handling for FoldAtenSplatPattern
6. removes a bug with aten.unflatten.int decomposition incorrectly generating a constant size int from a dynamic shape.
7. simplifies the dim list for aten.unflatten.int ops generated from the aten.view canonicalization in scalarize shapes.

All of these changes were necessary to unblock iree-org/iree#18899.

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