Before this change, the backend was unable to fold a build_vector dag
node with UNDEF operands into a single horizontal add/sub.
This patch teaches how to combine a build_vector with UNDEF operands into a
horizontal add/sub when possible. The algorithm conservatively avoids to combine
a build_vector with only a single non-UNDEF operand.
Added test haddsub-undef.ll to verify that we correctly fold horizontal binop
even in the presence of UNDEFs.
llvm-svn: 211265
Andrea Di Biagio