[llvm] [InlineSpiller] Drop unused elements in Virt2SiblingsMap. NFC (PR #147866)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm requested changes to this pull request.

I don't understand this. 0 isn't an invalid register index, it's the first valid virtual register index. 

https://github.com/llvm/llvm-project/pull/147866