Fix a problem in convert op rewriting where it used the original index for
ToIndicesOp.
Extend the concatenate op rewriting to handle dense destination and dynamic
shape destination.
Make the concatenate op integration test run on the codegen path.
nit: I learned this from Alex, who suggest me not specify the number of stack elements in smallvector (unless you have strong reasons for it), maybe we should start to follow the new practice.