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

This reverts commit adfea33.

Author: lei137

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

@@ -1,6 +1,5 @@
 ; 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() {