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

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

> The name "spillable" seems odd to me.

perhaps "foldable" is better? "memory foldable" ? "may spill", etc?

> I usually only talk about registers being spilled

Only `MachineOperand`s that are registered may be marked `spillable`. They _are_ registers; and only registers.

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