[DAGCombiner] Fold step_vector with add/mul/shl

This patch implements some DAG combines for STEP_VECTOR:
add step_vector(C1), step_vector(C2) -> step_vector(C1+C2)
add (add X step_vector(C1)), step_vector(C2) -> add X step_vector(C1+C2)
mul step_vector(C1), C2 -> step_vector(C1*C2)
shl step_vector(C1), C2 -> step_vector(C1<<C2)

TestPlan: check-llvm

Differential Revision: https://reviews.llvm.org/D100088