Merging r295213:

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r295213 | mkuper | 2017-02-15 10:37:26 -0800 (Wed, 15 Feb 2017) | 10 lines

[DAG] Don't try to create an INSERT_SUBVECTOR with an illegal source

We currently can't legalize those, but we should really not be creating
them in the first place, since legalization would probably look similar to the
way we legalize CONCAT_VECTORS - basically replace the INSERT with a BUILD.

This fixes PR311956.

Differential Revision: https://reviews.llvm.org/D29961

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llvm-svn: 295374