[X86] Improved target combine rules for selecting horizontal add/sub.

This patch slightly changes the algorithm introduced at revision 210477
to fix a problem where the algorithm was producing incorrect code for 
the VEX.256 encoded versions of horizontal add/sub.

For these cases, we now try to split the two 256-bit vectors into
128-bit chunks before emitting horizontal add/sub dag nodes.

Added a new test case into haddsub-2.ll.

llvm-svn: 210545