Further simplify CASE ... WHEN expressions #17589

CASE 
  WHEN <expr> THEN <expr>
  WHEN <expr> THEN <expr>
  ...
   ELSE
END

CASE 
  WHEN x=y THEN 1 
  WHEN true THEN 2 
  ELSE 3    -- can not be hit
END

CASE 
  WHEN x=y THEN 1 
  ELSE 2 
END

CASE 
  WHEN x=y THEN 1 
  WHEN y=z THEN 2 
  WHEN true THEN 3 
  WHEN a=b THEN 5 -- can not be hit
  ELSE 5    -- can not be hit
END

CASE 
  WHEN x=y THEN 1 
  WHEN y=z THEN 2 
  ELSE 3
END

> explain select CASE WHEN true then 1 else 2 END;
+---------------+-------------------------------+
| plan_type     | plan                          |
+---------------+-------------------------------+
| physical_plan | ┌───────────────────────────┐ |
|               | │       ProjectionExec      │ |
|               | │    --------------------   │ |
|               | │  CASE WHEN Boolean(true)  │ |
|               | │     THEN Int64(1) ELSE    │ |
|               | │        Int64(2) END:      │ |
|               | │             1             │ |
|               | └─────────────┬─────────────┘ |
|               | ┌─────────────┴─────────────┐ |
|               | │     PlaceholderRowExec    │ |
|               | └───────────────────────────┘ |
|               |                               |
+---------------+-------------------------------+
1 row(s) fetched.
Elapsed 0.002 seconds.