Merging r310552:

Merging r310552:
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r310552 | eladcohen | 2017-08-10 00:44:23 -0700 (Thu, 10 Aug 2017) | 19 lines

[SelectionDAG] When scalarizing vselect, don't assert on
a legal cond operand.

When scalarizing the result of a vselect, the legalizer currently expects
to already have scalarized the operands. While this is true for the true/false
operands (which have the same type as the result), it is not case for the
condition operand. On X86 AVX512, v1i1 is legal - this leads to operations such
as '< N x type> vselect < N x i1> < N x type> < N x type>' where < N x type > is
illegal to hit an assertion during the scalarization.

The handling is similar to r205625.
This also exposes the fact that (v1i1 extract_subvector) should be legal
and selectable on AVX512 - We do this by custom lowering to vector_extract_elt.
This still leaves us in some cases with redundant dag nodes which will be
combined in a separate soon to come patch.

This fixes pr33349.

Differential revision: https://reviews.llvm.org/D36511
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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_50@310635 91177308-0d34-0410-b5e6-96231b3b80d8