[mlir] Introduce AlignmentAttrOpInterface to expose MaybeAlign

[jiang1997]

This is the second part of the PRs addressing issue #155677.

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[jiang1997]

Hi @amd-eochoalo,
While grepping for remaining alignment uses, I noticed that most of the call sites aren't the op constructors themselves (as mentioned here), but rather the various lowering/transform passes that extract alignment using getAlignment() directly. For example:

• mlir/lib/Conversion/MemRefToEmitC/MemRefToEmitC.cpp
• mlir/lib/Conversion/MemRefToLLVM/MemRefToLLVM.cpp
• mlir/lib/Conversion/MemRefToSPIRV/MemRefToSPIRV.cpp
• mlir/lib/Conversion/SPIRVToLLVM/SPIRVToLLVM.cpp
• mlir/lib/Conversion/VectorToSPIRV/VectorToSPIRV.cpp
• mlir/lib/Dialect/MemRef/Transforms/EmulateNarrowType.cpp
• mlir/lib/Dialect/MemRef/Transforms/EmulateWideInt.cpp
• mlir/lib/Dialect/MemRef/Transforms/ExpandRealloc.cpp
• mlir/lib/Dialect/MemRef/Transforms/FlattenMemRefs.cpp
• mlir/lib/Dialect/SPIRV/IR/SPIRVDialect.cpp
• mlir/lib/Dialect/Vector/Transforms/LowerVectorGather.cpp

Do you think those call sites should also migrate to getMaybeAlign() as part of this cleanup, or should we keep this change scoped strictly to the constructor signatures for now?

[amd-eochoalo]

@jiang1997 thanks for the contribution! Let's keep this PR small and just focus on the changes you have at the moment :)