[LLVMdev] PerformDAGCombine vs. DAG to DAG

[Steve King]

Hello LLVM,
If there are any, can someone please explain rules of thumb for when
to do a PerformDAGCombine operation in ISelLowering vs. when to do a
DAG to DAG transformation?

I'm specifically thinking of an AND + SRL merge into a bit field
extract type instruction.  I see that the ARM target does this in
DAG-to-DAG, but this is literally a combine of two instructions,so why
not DAGCombine?

Thanks,
-steve