[ndk] Fix createBranchWeights() assertions.

Backport r196845 from the upstream to fix http://b.android.com/57381
The original commit message:

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commit cb3badc8949fb15ff5f812774c3ff9e698850f16
Author: Justin Bogner <mail   justinbogner com>
Date:   Tue Dec 10 00:13:41 2013 +0000

    Transforms: Don't create bad branch weights when folding a switch

    This avoids creating branch weight metadata of length one when we fold
    cases into the default of a switch instruction, which was triggering
    an assert.

    git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196845 91177308-0d3
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