[EarlyCSE] allow flexibility in atan(-0.0) test

[spatel]

Summary

As discussed in the post-commit feedback for b53d44fe47413c87f619b, this test was failing on AIX because atan(-0.0) results in 0.0 (positive).
I don't have an AIX system to test this pre-commit, so posting for review in case we can check that first.

[Jake-Egan]

Verified that it passes on AIX. LGTM.

llvm/test/Transforms/EarlyCSE/atan.ll

-; XFAIL: system-aix
-; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt -early-cse -S < %s | FileCheck %s
 
+; We allow either sign to provide flexibility for mathlib
+; implementations. The POSIX standard is not strict here.
+
 define float @callatan0() {
 ; CHECK-LABEL: @callatan0(
-; CHECK-NEXT:    ret float -0.000000e+00
+; CHECK-NEXT:    ret float {{-?}}0.000000e+00
 ;
   %call = call float @atanf(float -0.0)
   ret float %call
 }
 
 ; TODO: constant should be folded
 define float @callatanInf() {
 ; CHECK-LABEL: @callatanInf(