[llvm] [InlineAsm] Steal a bit to denote a register is spillable (PR #70738)

[via llvm-commits]

https://github.com/qcolombet approved this pull request.

In this context, I don't have an issue with calling this spillable as it matches the `LiveInterval` naming `markNotSpillable`.
I let you guys sort out the naming, but this LGTM otherwise.

In other words, don't merge before @MatzeB and you agree.

https://github.com/llvm/llvm-project/pull/70738