Remove virtual method IceOperand::setUse().

Keep it (non-virtual) in IceVariable, and use llvm::dyn_cast<> to find
it.
Remove IceOperand kinds Variable{Virtual,Physical}Register.

Physical registers are now represented as precolored IceVariables.
Update comment, missed in previous commit.
Make IceOperand::dump() non-virtual.

This removes the last of the virtual methods from the IceOperand
hierarchy, eliminating the vtable pointer and reducing the object
size.