[LoopUnroll+LoopUnswitch] do not transform loops containing callbr

[LoopUnroll+LoopUnswitch] do not transform loops containing callbr

Summary:
There is currently a correctness issue when unrolling loops containing
callbr's where their indirect targets are being updated correctly to the
newly created labels, but their operands are not.  This manifests in
unrolled loops where the second and subsequent copies of callbr
instructions have blockaddresses of the label from the first instance of
the unrolled loop, which would result in nonsensical runtime control
flow.

For now, conservatively do not unroll the loop.  In the future, I think
we can pursue unrolling such loops provided we transform the cloned
callbr's operands correctly.

Such a transform and its legalities are being discussed in:
https://reviews.llvm.org/D64101

Link: https://bugs.llvm.org/show_bug.cgi?id=42489
Link: https://groups.google.com/forum/#!topic/clang-built-linux/z-hRWP9KqPI
Reviewers: fhahn, hfinkel, efriedma

Reviewed By: fhahn, hfinkel, efriedma

Subscribers: efriedma, hiraditya, zzheng, dmgreen, llvm-commits, pirama, kees, nathanchance, E5ten, craig.topper, chandlerc, glider, void, srhines

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64368

llvm-svn: 366130