Auto merge of #147654 - dianqk:simplify-const-condition, r=cjgillot

Simplify trivial constants in SimplifyConstCondition

After `InstSimplify-after-simplifycfg` with `-Zub_checks=false`, there are many of the following patterns.

```
_13 = const false;
assume(copy _13);
_12 = unreachable_unchecked::precondition_check() -> [return: bb1, unwind unreachable];
```

Simplifying them to unreachable early should make CFG simpler.

Author: bors