Consolidate suboptimal usages of which() under one linter

[IndrajeetPatil]

Closes #1544

[AshesITR]

Isn't which.min(x) only the first index of a minimum value?

x <- c(1, 1)
which(x == min(x))
which.min(x)

[IndrajeetPatil]

Oh damn, you are right!
I didn't consider that using which(x == min(x)) to find all minimum elements, and not just the (first) one, is also a valid and intentional usage.

I am not sure how to proceed then. Just looking at this code then we can't guess the intent, and it's not possible to reliably suggest an alternative.

[AshesITR]

I would forfeit this linter unless we find a more efficient and still equivalent function. It is algorithmically possible with a single pass of x, but I'm not aware of any implementations.