Reorgnaize more switch code lowering to clean up some tricky

Reorgnaize more switch code lowering to clean up some tricky
code, and to eliminate the need for the SelectionDAGBuilder
state to be live during CodeGenAndEmitDAG calls.

Call SDB->clear() before CodeGenAndEmitDAG calls instead of
before it, and move the CurDAG->clear() out of SelectionDAGBuilder,
which doesn't own the DAG, and into CodeGenAndEmitDAG.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@102814 91177308-0d34-0410-b5e6-96231b3b80d8