It was excluded from spilling in a263a60, possibly by accident.
In the linked bug, we hit a situation like this:
```
%s = call @llvm.coro.suspend(...)
|
switch (%s)
case v1: / \ case v2:
... ...
| suspend point
| ...
\ /
%x = phi [v1] [v2]
|
...
|
use(%x)
```
Instcombine will notice that %x correlates exactly with %s, and so
use(%x) becomes use(%s).
However, corosplit would substitute different values for %s when
splitting the function, so even though %s had a particular value when
control actually passed through the switch, it could have a *different*
value when reaching use(%s).
This illustrates that while IR from the frontend typically does not use
these suspend return values across suspend points, mid-level
optimizations on the presplit coroutine may introduce new uses of
suspend values, so they must be considered eligible to spill to the
coroutine frame.
Fixes: #130326
00c527abab