Added F_UNDEFINEDP as an opcode

Also added a convenience SVAL(X) macro that returns the offset to
different parts of the svalue X values from the base register.

Also added some usage of the SVAL function in a few places.

Chagned to 32-bit arithmetics for types in a few locations. It saves
on code size, if nothing else (no REX prefix unless R8.. is used)