This patch teaches the DAGCombiner how to fold a sext/aext/zext dag node when
the operand in input is a build vector of constants (or UNDEFs).
The inability to fold a sext/zext of a constant build_vector was the root
cause of some pcg bugs affecting vselect expansion on x86-64 with AVX support.
Before this change, the DAGCombiner only knew how to fold a sext/zext/aext of a
ConstantSDNode.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200234 91177308-0d34-0410-b5e6-96231b3b80d8
