[mlir] Align mlir::Block (#106717)

This fixes an error from the LatticeAnchor PointerUnion with
ProgramPoint in b6603e1

```
third_party/llvm/llvm-project/llvm/include/llvm/ADT/PointerIntPair.h:172:17: error: static assertion failed due to requirement '2U <= PointerUnionUIntTraits<mlir::GenericLatticeAnchor *, mlir::ProgramPoint, mlir::Value>::NumLowBitsAvailable': PointerIntPair with integer size too large for pointer
  172 |   static_assert(IntBits <= PtrTraits::NumLowBitsAvailable,
```

Author: JoelWee

mlir/include/mlir/IR/Block.h

@@ -27,8 +27,8 @@ template <typename ValueRangeT>
 class ValueTypeRange;
 
 /// `Block` represents an ordered list of `Operation`s.
-class Block : public IRObjectWithUseList<BlockOperand>,
-              public llvm::ilist_node_with_parent<Block, Region> {
+class alignas(8) Block : public IRObjectWithUseList<BlockOperand>,
+                         public llvm::ilist_node_with_parent<Block, Region> {
 public:
   explicit Block() = default;
   ~Block();