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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> However, the `getPreSplitReg` function returns 0, register number rather than register index, when encountering a register that was not created by splitting. 

As in Register()? You shouldn't be checking against .id() == 0 for that case, rely on bool conversion? 


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