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[LoopUnswitch] Improve loop unswitch pass to find trivial unswitch conditions…

Description

[LoopUnswitch] Improve loop unswitch pass to find trivial unswitch conditions more effectively

Summary:
This patch improves trivial loop unswitch.

The current trivial loop unswitch only checks if loop header's terminator contains a trivial unswitch condition. But if the loop header only has one reachable successor (due to intentionally or unintentionally missed code simplification), we should consider the successor as part of the loop header. Therefore, instead of stopping at loop header's terminator, we should keep traversing its successors within loop until reach a *real* conditional branch or switch (whose condition can not be constant folded). This change will enable a single -loop-unswitch pass to unswitch multiple trivial conditions (unswitch one trivial condition could open opportunity to unswitch another one in the same loop), while the old implementation can unswitch only one per pass.

Reviewers: reames, broune

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11481