[mlir] Use `let arguments =` syntax instead of inheritance in LLVM dialect

LLVM dialect has been defining Op arguments by deriving the `Arguments` ODS
class. This has arguably worse readability due to large indentation caused by
multiple derivations, and is inconsistent with other ODS files. Use the `let
arguments` form instead.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D89560