Thanks to jacobly0 for figuring this out. The chain of events causing
the failure this triggered is as follows.
* As of a recent commit, certain bodies no longer emit a redundant
`block`, meaning there are more likely to be "interesting"
instructions (i.e. not blocks) at the end of parent GenZir scopes.
* When emitting the first `dbg_stmt` in such a body, the elision logic
incorrectly looks at a tag from an instruction in an enclosing scope.
* The tag of this instruction may be `undefined`, meaning that in unsafe
builds it may be incorrectly identified as a `dbg_stmt` instruction.
* This instruction from another body is clobbered rather than emitting
an actual `dbg_stmt` instruction. Note that this does not produce
invalid ZIR, since the creator of the undefined instruction replaces
the previously-undefined payload later.
6a87e42c2e