general: Replace unreachable-imitating assertions with UNREACHABLE()

We can just use the self-documenting assertion for indicating
unreachable paths, instead of manually passing false and providing a
message.

src/backend/x64/emit_x64.cpp

@@ -128,7 +128,7 @@ void EmitX64::EmitGetNZCVFromOp(EmitContext& ctx, IR::Inst* inst) {
         case IR::Type::U64:
             return 64;
         default:
-            ASSERT_MSG(false, "Unreachable");
+            UNREACHABLE();
             return 0;
         }
     }();