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

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csstormq wrote:

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

@arsenm , thanks for your review. In my opinion, the key of `Virt2SiblingsMap` (the original register) should always hold the equal value as its corresponding value (the sibling register). However, the `getPreSplitReg` function returns 0, register number rather than register index, when encountering a register that was not created by splitting. Consequently, `Virt2SiblingsMap[0]` ends up storing registers with differing values, which directly contradicts the map's design principle.

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