Merging r339600:

Merging r339600:
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r339600 | scott.linder | 2018-08-13 20:44:21 +0200 (Mon, 13 Aug 2018) | 8 lines

[CodeGen] Fix assert in SelectionDAG::computeKnownBits

Fix SelectionDAG::computeKnownBits asserting when handling EXTRACT_SUBVECTOR
when zero extending the demanded elements mask if it is already as long as the
source vector.

Differential Revision: https://reviews.llvm.org/D49574

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_70@339664 91177308-0d34-0410-b5e6-96231b3b80d8