[PowerPC][AIX] xfail atan-intrinsic to unblock bot (#143723)

Testcase from #143416 is
causing the AIX bot to be red. XFAIL for now till issue can be resolved.

Author: lei137

llvm/test/Transforms/InstSimplify/ConstProp/atan-intrinsic.ll

@@ -1,5 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
 ; RUN: opt -S -passes=instsimplify < %s | FileCheck %s
+; XFAIL: target={{.*}}-aix{{.*}}
 
 define double @test_atan_0() {
 ; CHECK-LABEL: define double @test_atan_0() {