[WebAssembly] Set unreachable as canonical to permit disassembly

Currently, using llvm-objdump to disassemble a function containing
unreachable will trigger an assertion while decoding the opcode, since both
unreachable and debug_unreachable have the same encoding. To avoid this, set
unreachable as the canonical decoding.

Differential Revision: https://reviews.llvm.org/D87431