[PR27599] [SystemZ] [SelectionDAG] Fix extension of atomic cmpxchg result.

[PR27599] [SystemZ] [SelectionDAG] Fix extension of atomic cmpxchg result.

Currently, SelectionDAG assumes 8/16-bit cmpxchg returns either a sign
extended result, or a zero extended result.  SystemZ takes a third
option by returning junk in the high bits (rotated contents of the other
bytes in the memory word).  In that case, don't use Assert*ext, and
zero-extend the result ourselves if a comparison is needed.

Differential Revision: http://reviews.llvm.org/D19800

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@269075 91177308-0d34-0410-b5e6-96231b3b80d8