[mlir][Vector] Add vector.to_elements op (#141457)

This PR introduces the `vector.to_elements` op, which decomposes a
vector into its scalar elements. This operation is symmetrical to the
existing `vector.from_elements`.

Examples:

```
    // Decompose a 0-D vector.
    %0 = vector.to_elements %v0 : vector<f32>
    // %0 = %v0[0]

    // Decompose a 1-D vector.
    %0:2 = vector.to_elements %v1 : vector<2xf32>
    // %0#0 = %v1[0]
    // %0#1 = %v1[1]

    // Decompose a 2-D.
    %0:6 = vector.to_elements %v2 : vector<2x3xf32>
    // %0#0 = %v2[0, 0]
    // %0#1 = %v2[0, 1]
    // %0#2 = %v2[0, 2]
    // %0#3 = %v2[1, 0]
    // %0#4 = %v2[1, 1]
    // %0#5 = %v2[1, 2]
```

This op is aimed at reducing code size when modeling "structured" vector
extractions and simplifying canonicalizations of large sequences of
`vector.extract` and `vector.insert` ops into `vector.shuffle` and other
sophisticated ops that can re-arrange vector elements.

mlir/tools/mlir-tblgen/OpFormatGen.cpp

@@ -2787,6 +2787,11 @@ private:
   void handleTypesMatchConstraint(
       StringMap<TypeResolutionInstance> &variableTyResolver, const Record &def);
 
+  /// Check for inferable type resolution based on
+  /// `ShapedTypeMatchesElementCountAndTypes` constraint.
+  void handleShapedTypeMatchesElementCountAndTypesConstraint(
+      StringMap<TypeResolutionInstance> &variableTyResolver, const Record &def);
+
   /// Returns an argument or attribute with the given name that has been seen
   /// within the format.
   ConstArgument findSeenArg(StringRef name);
@@ -2850,6 +2855,9 @@ LogicalResult OpFormatParser::verify(SMLoc loc,
       handleSameTypesConstraint(variableTyResolver, /*includeResults=*/true);
     } else if (def.isSubClassOf("TypesMatchWith")) {
       handleTypesMatchConstraint(variableTyResolver, def);
+    } else if (def.isSubClassOf("ShapedTypeMatchesElementCountAndTypes")) {
+      handleShapedTypeMatchesElementCountAndTypesConstraint(variableTyResolver,
+                                                            def);
     } else if (!op.allResultTypesKnown()) {
       // This doesn't check the name directly to handle
       //    DeclareOpInterfaceMethods<InferTypeOpInterface>
@@ -3289,6 +3297,24 @@ void OpFormatParser::handleTypesMatchConstraint(
     variableTyResolver[rhsName] = {arg, transformer};
 }
 
+void OpFormatParser::handleShapedTypeMatchesElementCountAndTypesConstraint(
+    StringMap<TypeResolutionInstance> &variableTyResolver, const Record &def) {
+  StringRef shapedArg = def.getValueAsString("shaped");
+  StringRef elementsArg = def.getValueAsString("elements");
+
+  // Check if the 'shaped' argument is seen, then we can infer the 'elements'
+  // types.
+  if (ConstArgument arg = findSeenArg(shapedArg)) {
+    variableTyResolver[elementsArg] = {
+        arg, "::llvm::SmallVector<::mlir::Type>(::llvm::cast<::mlir::"
+             "ShapedType>($_self).getNumElements(), "
+             "::llvm::cast<::mlir::ShapedType>($_self).getElementType())"};
+  }
+
+  // Type inference in the opposite direction is not possible as the actual
+  // shaped type can't be inferred from the variadic elements.
+}
+
 ConstArgument OpFormatParser::findSeenArg(StringRef name) {
   if (const NamedTypeConstraint *arg = findArg(op.getOperands(), name))
     return seenOperandTypes.test(arg - op.operand_begin()) ? arg : nullptr;