* 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.
125b85d737