[llvm] [CodeGen] Let `PassBuilder` support machine passes (PR #76320)

[Arthur Eubanks via llvm-commits]

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@@ -0,0 +1,499 @@
+//===- unittests/MIR/PassBuilderCallbacksTest.cpp - PB Callback Tests --===//
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aeubanks wrote:

ultra nit: line this up

https://github.com/llvm/llvm-project/pull/76320