[SimplifyCFG] propagate branch metadata when creating select (retry r268550 /...

[SimplifyCFG] propagate branch metadata when creating select (retry r268550 / r268751 with possible fix)

Retrying r268550/r268751 which were reverted at r268577/r268765 due a memory sanitizer failure.
I have not been able to reproduce that failure, but I've taken another guess at fixing
the problem in this version of the patch and will watch for another failure.

Original commit message:
Unlike earlier similar fixes, we need to recalculate the branch weights
in this case.

Differential Revision: http://reviews.llvm.org/D19674

llvm-svn: 268767