This patch refactors 'noce_try_store_flag_constants' a bit to make it easier to
reason about.

The function contains two series of conditions, and each branch of the second
series corresponds to one or two branches of the first series. The patch
introduces a new enumeration strategy_t instead and uses it to select the
correct branch. Also, ISTM that the last 'else' branch is unreachable.

Bootstrapped and regtested on x86_64-linux. OK for trunk?

-- 
Regards,
    Mikhail Maltsev

gcc/ChangeLog:

2016-05-23  Mikhail Maltsev  <maltsevm@gmail.com>

        * ifcvt.c (noce_try_store_flag_constants): Refactor.