[MLIR][memref] Add details on the semantic of reinterpret_cast

[qcolombet]

Summary

Make it clearer what the semantic of reinterpret_cast is.

In particular, call out that this instruction is not a no-op.

NFC

Related to https://github.com/llvm/llvm-project/issues/59896

[pifon2a]

Thank you!

[mehdi_amini]

mlir/include/mlir/Dialect/MemRef/IR/MemRefOps.td

1259

In general I believe we avoided defining the memref semantics in terms of "descriptor" which was intended as one possible LLVM lowering (so an implementation detail basically).

(I'm not sure if the doc is still decoupled though)

[qcolombet]

mlir/include/mlir/Dialect/MemRef/IR/MemRefOps.td

1259

Ah shoot, I already pushed the change.
How do you recommend that we amend this change?

Leaving out the "In other words[...]" part that breaks down the memref descriptor?

mlir/include/mlir/Dialect/MemRef/IR/MemRefOps.td

let description = [{
     : memref<?x?xf32> to memref<?x10xf32, strided<[1, ?], offset: 0>>
 
     memref.reinterpret_cast %unranked to
       offset: [%offset],
       sizes: [%size0, %size1],
       strides: [%stride0, %stride1]
     : memref<*xf32> to memref<?x?xf32, strided<[?, ?], offset: ?>>
     ```
+
+    This operation creates a new memref descriptor using the base of the
+    source and applying the input arguments to the other metadata.
+    In other words:
+    ```mlir
+    %dst = memref.reinterpret_cast %src to
+      offset: [%offset],
+      sizes: [%sizes],
+      strides: [%strides]
+    ```
+    means that `%dst`'s descriptor will be:
+    ```mlir
+    %dst.base = %src.base
+    %dst.aligned = %src.aligned
+    %dst.offset = %offset
+    %dst.sizes = %sizes
+    %dst.strides = %strides
+    ```
   }];
 
   let arguments = (ins Arg<AnyRankedOrUnrankedMemRef, "", []>:$source,
                        Variadic<Index>:$offsets,
                        Variadic<Index>:$sizes,
                        Variadic<Index>:$strides,
                        DenseI64ArrayAttr:$static_offsets,
                        DenseI64ArrayAttr:$static_sizes,