[SystemZ] Mark CC defs as dead whenever possible.

Marking implicit CC defs as dead everywhere except when CC is actually
defined and used explicitly, is important since the post-ra scheduler
will otherwise insert edges between instructions unnecessarily.

Also temporarily disable LA(Y)-> AGSI optimization in
foldMemoryOperandImpl(), since this inroduces a def of the CC reg,
which is illegal unless it is known to be dead.

Reviewed by Ulrich Weigand.

llvm-svn: 268215