[llvm] [X86] Skip unused VRegs traverse (PR #78229)

[Evgenii Kudriashov via llvm-commits]

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@@ -860,3 +864,63 @@ body:             |
     RET 0
 
 ...
+---
+name:            test_globalisel_unused
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e-kud wrote:

Even if it is present, it has no impact, as we run a single pass and MIR has been already selected. I want to show here that MIR for this function is obtained using GlobalISel, as a result we have registers without classes `{ id: 0, class: _, preferred-register: '' }`. Yeah, I think more detailed comment will be better. Or do you think it may be even better to extract it to a separate file? 

https://github.com/llvm/llvm-project/pull/78229