Added local optimizations: Mux((a != b, 0, a op b) => a op b.

This works for Eor and Sub.
Sharpen relation of Confirm nodes.
Improved local optimization: a - a => 0.
Improved local optimization: a ^ a => 0.
Check whether mode_b is already lowered.
remove type_not_visited() and use !type_visited() instead
remove inline version of {set,inc}_master_type_visited()

Both functionsa re not performance sensitive.
do not use identifiers with underscore prefix

Underscore prefix is reserved by the C language standard.
set type alignment at type construction time
ia32: small structs are returned in int regs on darwin