[RFC] Cleaning up latch blocks

[Revital1 Eres <ERES at il dot ibm dot com>]

Hello,

SMS currently works only on single-basic-block loops.  This simplifies
the task of software pipelining.  PR34263 is an example where outof-ssa
creates a non-empty latch block for a single-basic-block loop and thus
prevents SMS to be applied on it. This issue was raised in the past
(http://gcc.gnu.org/ml/gcc-patches/2005-11/msg01971.html) but I am not
sure what is the best approach to address it. Cleaning-up those latch
blocks for the propose of restoring the single-basic-block loop should
be helpful in general (and not only in the SMS perspective).

So, we can address this inside SMS or alternatively in outof-ssa as in the
attached patch (which was originally written by Andrew Pinski for PR19038
and rewritten by Mircea, it also passes bootstrap and regtest on ppc64).

Thanks,
Revital

(See attached file: patch_empty_latch_2_12.txt)

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