Previously `@as(i64, undefined) +% 1` would produce `@as(@TypeOf(undefined), undefined)` which now gives `@as(i64, undefined)`.
Previously `@as(i64, undefined) +| 1` would hit an assertion which now gives `@as(i64, undefined)`.
Many of these tests check for the incorrect behavior of stage1 whereas
self-hosted correctly does not emit an error, so they are simply
deleted.
The remaining number of test cases within the stage1/ subdirectory is
reduced from 143 to 103.