[TIR] Minor refactor to tir.transform.StorageFlatten (#9260)
[TIR] Minor refactor to tir.transform.StorageFlatten (#9260)

Expressed each step as a separate `transform::Pass`, so they can be
used/inspected individually.
dmlc/tvm,dmlc/tvm,Laurawly/tvm-1,Laurawly/tvm-1,dmlc/tvm,Laurawly/tvm-1,dmlc/tvm,Laurawly/tvm-1,dmlc/tvm,Laurawly/tvm-1,dmlc/tvm,dmlc/tvm,Laurawly/tvm-1,dmlc/tvm,Laurawly/tvm-1,dmlc/tvm,Laurawly/tvm-1,Laurawly/tvm-1,Laurawly/tvm-1