Correct codegen of `ConstValue::Indirect` scalar and scalar pair
This concerns 3 tricky cases with `ConstValue::Indirect`:
- if we want a non-pointer scalar;
- if we have non-zero offset;
- if offset points to uninit memory => generate `poison` instead of an ICE. This case could happen in unreachable code, trying to extract a field from the wrong variant.
Those cases are not currently emitted by the compiler, but are exercised by https://github.com/rust-lang/rust/pull/116012.
5899a80ae6