[x86] lower extracted add/sub to horizontal vector math

add (extractelt (X, 0), extractelt (X, 1)) --> extractelt (hadd X, X), 0

This is the integer sibling to D56011.

There's an additional restriction to only to do this transform in the
case where we don't have extra extracts from the source vector. Without
that, we can fail to match larger horizontal patterns that are more 
beneficial than this minimal case. An improvement to the more general 
h-op lowering may allow us to remove the restriction here in a follow-up.

llvm-svn: 351093