stage2-wasm: finishAir handle .stack result

By allowing finishAir to handle .stack results, we simplify a lot of code in
air*** functions, which try to handle this case. Also this changes will result in optimization, if one of operands is dead after instruction its place could be reused by result.

The only downside to this change is that finishAir now can return error, though it handled by returning finishAir result, because it always needs to be final in air*** functions.

Additionally I migrated WValue{ to .{ inside CodeGen.zig.

Author: pavelverigo