[WebAssembly] Set unreachable as canonical to permit disassembly

[ddcc]

Summary

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.

llvm/lib/Target/WebAssembly/WebAssemblyInstrControl.td

defm RETURN : I<(outs), (ins variable_ops), (outs), (ins),
                 "return", "return", 0x0f>;
 // Equivalent to RETURN, for use at the end of a function when wasm
 // semantics return by falling off the end of the block.
 let isCodeGenOnly = 1 in
 defm FALLTHROUGH_RETURN : I<(outs), (ins variable_ops), (outs), (ins), []>;
 
 } // isReturn = 1
 
-let isTrap = 1 in
+let IsCanonical = 1, isTrap = 1 in
 defm UNREACHABLE : NRI<(outs), (ins), [(trap)], "unreachable", 0x00>;
 
 } // isTerminator = 1
 
 // debugtrap explicitly returns despite trapping because it is supposed to just
 // get the attention of the debugger. Unfortunately, because UNREACHABLE is a
 // terminator, lowering debugtrap to UNREACHABLE can create an invalid
 // MachineBasicBlock when there is additional code after it. Lower it to this