[llvm] [NFC] Add `const` for readability (PR #116611)

[Han-Kuan Chen via llvm-commits]

HanKuanChen wrote:

> > The `copyFrom` for `llvm/include/llvm/CodeGen/LiveInterval.h` is irrelevant to const.
> 
> What do you mean? I make the reference parameter const.

Sorry, my bad.

https://github.com/llvm/llvm-project/pull/116611