[NFC][GlobalISel] Don't return `bool` from apply functions

There is no case where those functions return false. It's always return true.
Even if they were to return false, it's not really something we should rely on I think.
With the current combiner implementation, it would just make `tryCombineAll` return false without retrying anymore rules.

I also believe that if an applyer were to return false, it would mean that the match function is not good enough. Asserting on failure in an apply function is a better idea, IMO.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D153619