AArch64ISel: Fix ccmp code selection matching deep expressions.

Some of the conditions necessary to produce ccmp sequences were only
checked in recursive calls to emitConjunctionDisjunctionTree() after
some of the earlier expressions were already built. Move all checks over
to isConjunctionDisjunctionTree() so they are all checked before we
start emitting instructions.

Also rename some variable to better reflect their usage.

llvm-svn: 258605