[Bug ipa/95558] Invalid IPA optimizations based on weak definition

[bugdal at aerifal dot cx]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95558

--- Comment #2 from Rich Felker <bugdal at aerifal dot cx> ---
Wow. It's interesting that we've never seen this lead to incorrect codegen
before, though. All weak dummies should be affected, but only in some cases
does the pure get used to optimize out the external call.

This suggests there's a major missed optimization around pure functions too, in
addition to the wrong application of pure (transfering it from the weak
definition to the external declaration) that's the bug.