Make the code slightly shorter, faster, and easier to
read.

* Eliminate unused DUP_TOPX code for x==1.
compile.c always generates DUP_TOP instead.

* Since only two cases remain for DUP_TOPX, replace
the switch-case with if-elseif.

* The in-lined integer compare does a CheckExact on
both arguments. Since the second is a little more
likely to fail, test it first.

* The switch-case for IS/IS_NOT and IN/NOT_IN can
separate the regular and inverted cases with no
additional work. For all four paths, saves a test and
jump.