Remove possible loop creation in DAGCombiner

[deadal nix]

DAGCombiner can create loop in the DAG when you sext the result of a setcc,
depending on the target.

This diff remove the call to DAG.getSExtOrTrunc that create the loop. This
call used to be necessary for x86 backend, but it doesn't look necessary
anymore. I've added tests to ensure it doesn't.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20140924/d3b787f9/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: 0001-Remove-loop-generation-in-DAGCombiner.patch
Type: text/x-patch
Size: 2271 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20140924/d3b787f9/attachment.bin>