There are some <unexpected> instructions in textual semir

[danakj]

Description of the bug:

Some instructions have snuck into our test cases. We should figure out what instructions they reference and get those instructions to be in a block that is included in formatted semir.

Example in toolchain/check/testdata/alias/fail_control_flow.carbon: https://github.com/carbon-language/carbon-lang/pull/5361/files#diff-82254f33573e9d6dcdb65434312087f4513fc2e554290ed0309575daf9174814R80

[jonmeow]

We should be concerned if these show up in valid code, but particularly for erroneous code, there may not be a good way to put instructions in a valid control flow that the formatter fully recognizes for naming.