Merging r298604:

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r298604 | niravd | 2017-03-23 11:01:50 -0400 (Thu, 23 Mar 2017) | 14 lines

[SDAG] Fix zeroExtend assertion error

Move CombineTo preventing deleted node from being returned in
visitZERO_EXTEND.

Fixes PR32284.

Reviewers: RKSimon, bogner

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31254
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llvm-svn: 300979