Merging r226029:

Merging r226029:
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r226029 | dexonsmith | 2015-01-14 11:56:10 -0800 (Wed, 14 Jan 2015) | 7 lines

IR: Fix a use-after-free in RAUW

Happened pretty commonly during `LLVMContext` teardown when `clang -g`
hit an error.  This fixes the use-after-free.  Next I'll clean up
teardown so that it's not RAUW'ing when metadata-tracked values are
deleted (only really causes a problem if the graph is mid-construction
when teardown starts, but it's still unnecessary work).
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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_36@226090 91177308-0d34-0410-b5e6-96231b3b80d8