Merging r304537:

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r304537 | john.brawn | 2017-06-02 06:24:14 -0400 (Fri, 02 Jun 2017) | 9 lines

[GlobalMerge] Don't merge globals that may be preempted

When a global may be preempted it needs to be accessed directly, instead of
indirectly through a MergedGlobals symbol, for the preemption to work.

This fixes PR33136.

Differential Revision: https://reviews.llvm.org/D33727

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llvm-svn: 305187