[mlir][docs] Add usage/example of OpAsmOpInterface (llvm#123610)

This is part of
https://discourse.llvm.org/t/rfc-introduce-opasm-type-attr-interface-for-pretty-print-in-asmprinter/83792.

OpAsmOpInterface controls the SSA Name/Block Name and Default Dialect
Prefix. This PR adds the usage of them by existing examples in MLIR.

Author: ZenithalHourlyRate

mlir/docs/DefiningDialects/Assembly.md

@@ -48,4 +48,79 @@ void MyDialect::initialize() {
 ```
 
 * If `getAlias` provides an alias with a trailing digit, `AsmPrinter` appends an underscore to avoid conflicts with autogenerated IDs.
-* If multiple types/attributes have the same alias from `getAlias`, a number is appended to the alias to avoid conflicts.
+* If multiple types/attributes have the same alias from `getAlias`, a number is appended to the alias to avoid conflicts.
+
+## Suggesting SSA/Block Names
+
+An `Operation` can suggest the SSA name prefix using `OpAsmOpInterface`.
+
+For example, `arith.constant` will suggest a name like `%c42_i32` for its result:
+
+```tablegen
+include "mlir/IR/OpAsmInterface.td"
+
+def Arith_ConstantOp : Op<Arith_Dialect, "constant",
+    [ConstantLike, Pure,
+     DeclareOpInterfaceMethods<OpAsmOpInterface, ["getAsmResultNames"]>]> {
+...
+}
+```
+
+And the corresponding method:
+
+```cpp
+// from https://github.com/llvm/llvm-project/blob/5ce271ef74dd3325993c827f496e460ced41af11/mlir/lib/Dialect/Arith/IR/ArithOps.cpp#L184
+void arith::ConstantOp::getAsmResultNames(
+    function_ref<void(Value, StringRef)> setNameFn) {
+  auto type = getType();
+  if (auto intCst = llvm::dyn_cast<IntegerAttr>(getValue())) {
+    auto intType = llvm::dyn_cast<IntegerType>(type);
+
+    // Sugar i1 constants with 'true' and 'false'.
+    if (intType && intType.getWidth() == 1)
+      return setNameFn(getResult(), (intCst.getInt() ? "true" : "false"));
+
+    // Otherwise, build a complex name with the value and type.
+    SmallString<32> specialNameBuffer;
+    llvm::raw_svector_ostream specialName(specialNameBuffer);
+    specialName << 'c' << intCst.getValue();
+    if (intType)
+      specialName << '_' << type;
+    setNameFn(getResult(), specialName.str());
+  } else {
+    setNameFn(getResult(), "cst");
+  }
+}
+```
+
+Similarly, an `Operation` can suggest the name for its block arguments using `getAsmBlockArgumentNames` method in `OpAsmOpInterface`.
+
+For custom block names, `OpAsmOpInterface` has a method `getAsmBlockNames` so that
+the operation can suggest a custom prefix instead of a generic `^bb0`.
+
+## Defining Default Dialect
+
+An `Operation` can indicate that the nested region in it has a default dialect prefix, and the operations in the region could elide the dialect prefix.
+
+For example, in a `func.func` op all `func` prefix could be omitted:
+
+```tablegen
+include "mlir/IR/OpAsmInterface.td"
+
+def FuncOp : Func_Op<"func", [
+  OpAsmOpInterface
+  ...
+]> {
+  let extraClassDeclaration = [{
+    /// Allow the dialect prefix to be omitted.
+    static StringRef getDefaultDialect() { return "func"; }
+  }];
+}
+```
+
+```mlir
+func.func @main() {
+  // actually func.call
+  call @another()
+}
+```