[SystemZ] Set getExtendForAtomicOps to ISD::ANY_EXTEND

The implementation of subword atomics does not actually
guarantee the result is zero-extended, which now caused
build bot failures after https://reviews.llvm.org/D101342
was landed.

llvm/lib/Target/SystemZ/SystemZISelLowering.h

@@ -569,7 +569,7 @@ class SystemZTargetLowering : public TargetLowering {
                                            unsigned Depth) const override;
 
   ISD::NodeType getExtendForAtomicOps() const override {
-    return ISD::ZERO_EXTEND;
+    return ISD::ANY_EXTEND;
   }
   ISD::NodeType getExtendForAtomicCmpSwapArg() const override {
     return ISD::ZERO_EXTEND;