R600: Implement computeMaskedBitsForTargetNode for BFE
R600: Implement ComputeNumSignBitsForTargetNode for BFE
R600: Add dag combine for BFE
R600: Try to convert BFE back to standard bit ops when possible.

This allows existing DAG combines to work on them, and then
we can re-match to BFE if necessary during instruction selection.