stage2: error return tracing handles ret better

Sema: insert an error return trace frame when appropriate when analyzing
ret_load. Also optimize the instructions to avoid an unnecessary block
sent to the backends.

AstGen: always emit a dbg_stmt for return expressions, in between the
defer instructions and the return instruction.

This improves the following test case:

```zig
pub fn main() !void {
    return foo();
}

fn foo() !void {
    return error.Bad;
}
```

The error return trace now points to the return token instead of
pointing to the foo() function call, matching stage1.

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