move "unreachable code" error from stage1 to stage2

 * AstGen: implement "unreachable code" error for blocks. This works at
   the statement level.
 * stage1: remove the "unreachable code" error implementation, which
   means removing the `is_gen` field from IrInstSrc. This is one small
   step towards a smaller memory footprint for stage1. The benefits
   won't be realized until a future commit because this flag took
   advantage of padding.

There may be a regression here with "union has no associated enum"
error, and there is a regression with the following code:

```zig
const a = noreturn;
```

A future commit will address these regressions.

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