AArch64: Fix invalid CCMP emission

The code emitting AND-subtrees used to check whether any of the operands
was an OR in order to figure out if the result needs to be negated.
However the OR could be hidden in further subtrees and not immediately
visible.

Change the code so that canEmitConjunction() determines whether the
result of the generated subtree needs to be negated. Cleanup emission
logic to use this. I also changed the code a bit to make all negation
decisions early before we actually emit the subtrees.

This fixes http://llvm.org/PR39550

Differential Revision: https://reviews.llvm.org/D54137

llvm-svn: 348444