Moves all of the IR-related files (instruction generation, encoding, decoding, disassembly, instructions, operands, instruction lists) from core/arch to core/ir, mirroring the arch-specific subdirectories under core/ir. This is a code cleanup step toward properly isolating the drdecode library, as well as moving us toward the ability to build for a separate target architecture from the host architecture and eventually perhaps building in multiple target architectures in the same binary for decoding and IR manipulation.
Also renames mangle_utils.c to ir_utils.c to better fit its purpose and location.
Issue: #1684