[AMDGPU] pin lit test divergent-unswitch.ll to the old pass manager

The loop-unswitch transform should not be performed on a loop whose
condition is divergent. For this to happen correctly, divergence
analysis must be available. The existing divergence analysis has not
been ported to the new pass manager yet. As a result, loop unswitching
on the new pass manager is currently unsafe on targets that care about
divergence.

This test is temporarily disabled to unblock work on the new pass
manager. The issue is now tracked in bug 48819.

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D95051