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

[Jay Foad via llvm-commits]

jayfoad wrote:

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

OK, since @nikic also mentioned that in #66482, I'll do it.

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