[Flang] Xfail hlfir test case on AIX (#76802)

This test case seems to fail at the `Merge disjoint stack slots` pass on
AIX, it passes if compilled with `-mllvm --no-stack-coloring`. This PR
xfails the teest case on AIX temporarily until the issue is addressed.

---------

Co-authored-by: Mark Danial <[email protected]>

Author: madanial0

flang/test/HLFIR/simplify-hlfir-intrinsics.fir

@@ -1,3 +1,6 @@
+// XFail the following test case on AIX due to potential miscompilation
+// TODO: Crash fir-opt on AIX
+// XFAIL: system-aix
 // RUN: fir-opt --simplify-hlfir-intrinsics %s | FileCheck %s
 
 // box with known extents