PR16054: Slight strengthening for -Wsometimes-uninitialized: if we use a
variable uninitialized every time we reach its (reachable) declaration, or
every time we call the surrounding function, promote the warning from
-Wmaybe-uninitialized to -Wsometimes-uninitialized.

This is still slightly weaker than desired: we should, in general, warn
if a use is uninitialized the first time it is evaluated.