[llvm] Loosen DWARF form check in discriminant-member.ll (PR #139258)

[Hubert Tong via llvm-commits]

hubert-reinterpretcast wrote:

@tromey, you can reproduce this (without access to an AIX machine) by adding `-mtriple=powerpc64-ibm-aix` to the `llc` invocations.

https://github.com/llvm/llvm-project/pull/139258