[R600] Replicate old DAGCombiner behavior in target specific DAG combine.

build_vector is lowered to REG_SEQUENCE, which is something the register
allocator does a good job at optimizing.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187397 91177308-0d34-0410-b5e6-96231b3b80d8