[mlir] Ensure index attrs are always 64-bit APInts

[Mogball]

Summary

This patch ensures that index integer attributes can only be
constructed with APInts whose widths are equal to the index
internal storage bitwidth (64).

mlir/lib/IR/BuiltinAttributes.cpp

LogicalResult IntegerAttr::verify(function_ref<InFlightDiagnostic()> emitError,
                                   Type type, APInt value) {
   if (IntegerType integerType = type.dyn_cast<IntegerType>()) {
     if (integerType.getWidth() != value.getBitWidth())
       return emitError() << "integer type bit width (" << integerType.getWidth()
                          << ") doesn't match value bit width ("
                          << value.getBitWidth() << ")";
     return success();
   }
-  if (type.isa<IndexType>())
+  if (type.isa<IndexType>()) {
+    if (value.getBitWidth() != IndexType::kInternalStorageBitWidth)
+      return emitError()
+             << "value bit width (" << value.getBitWidth()
+             << ") doesn't match index type internal storage bit width ("
+             << IndexType::kInternalStorageBitWidth << ")";
     return success();
+  }
   return emitError() << "expected integer or index type";
 }
 
 BoolAttr IntegerAttr::getBoolAttrUnchecked(IntegerType type, bool value) {
   auto attr = Base::get(type.getContext(), type, APInt(/*numBits=*/1, value));
   return attr.cast<BoolAttr>();
 }