SelectionDAG: Teach the legalizer to split SETCC if VSELECT needs splitting too.

This patch reapplies r193676 with an additional fix for the Hexagon backend. The
SystemZ backend has already been fixed by r194148.

The Type Legalizer recognizes that VSELECT needs to be split, because the type
is to wide for the given target. The same does not always apply to SETCC,
because less space is required to encode the result of a comparison. As a result
VSELECT is split and SETCC is unrolled into scalar comparisons.

This commit fixes the issue by checking for VSELECT-SETCC patterns in the DAG
Combiner. If a matching pattern is found, then the result mask of SETCC is
promoted to the expected vector mask type for the given target. Now the type
legalizer will split both VSELECT and SETCC.

This allows the following X86 DAG Combine code to sucessfully detect the MIN/MAX
pattern. This fixes PR16695, PR17002, and <rdar://problem/14594431>.

Reviewed by Nadav

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194542 91177308-0d34-0410-b5e6-96231b3b80d8
R600: Fix selection failure on EXTLOAD

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194547 91177308-0d34-0410-b5e6-96231b3b80d8
Fix a null pointer dereference when copying a null polymorphic pointer.

This bug only bit the C++98 build bots because all of the actual uses
really do move. ;] But not *quite* ready to do the whole C++11 switch
yet, so clean it up. Also add a unit test that catches this immediately.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194548 91177308-0d34-0410-b5e6-96231b3b80d8
Add another (perhaps better) video for Sean's talk. (Thanks Marshall!)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194549 91177308-0d34-0410-b5e6-96231b3b80d8