Clean up dca code (using C99).
Delete strange switch cases.

We should never see an Id node and Call nodes should never have mode_X.
Fixed dca transfer function for Add, Sub, Minus, Mul and Shl.

We have to consider that less significant bits of the operands
can affect more significant bits of the operation itself.

This fixes opt/occult2.c and 177.mesa.
Improved dca transfer function for Shr and Shrs.