Merging r277114:

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r277114 | majnemer | 2016-07-28 22:39:21 -0700 (Thu, 28 Jul 2016) | 6 lines

[EarlyCSE] Correctly handle simplified, but live, instructions

Some instructions may have their uses replaced with a symbolic constant.
However, the instruction may still have side effects which percludes it
from being removed from the function.  EarlyCSE treated such an
instruction as if it were removed, resulting in PR28763.
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llvm-svn: 277382