Move some comparison simplifications to match.pd

[Marc Glisse]

On Tue, 27 Oct 2015, Kyrill Tkachov wrote:

> So here the types are shorts and unsigned shorts. On aarch64 these are 
> HImode values and there's no direct arithmetic operations on them, so 
> they have to be extended to SImode and truncated back.

Ah ok, that makes sense. I expect it is in the case where @0 is shorter 
than a word and @2 is longer than @0. I wouldn't expect the signedness to 
matter that much, but maybe one of sign/zero-extension simplifies better 
than the other.

This problem would happen because we are still missing a pass that handles 
type promotion.

It means we could consider gating the transformation by the existence of 
lt in the mode of @0, but I don't see any query of optabs in match.pd 
yet...

Still, a testcase would be necessary to go with whatever patch, and I may 
be completely on the wrong track.

-- 
Marc Glisse