[llvm] [update_mir_test_checks] Handle multiple defs of vreg (PR #66483)

[Nicolai Hähnle via llvm-commits]

https://github.com/nhaehnle commented:

Shouldn't the test demonstrate which CHECK lines are added? Other tests add a .expected file.

Apart from that, the change looks pretty reasonable to me.

https://github.com/llvm/llvm-project/pull/66483