[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

llvm/lib/Target/WebAssembly/WebAssemblyInstrControl.td

@@ -103,7 +103,7 @@ 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