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

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

> I'd be fine with AllowsMemoryAddressing or MemoryFoldable; I'd personally not add a Reg prefix to the name but feel free to add it if you think that is important.

For the sake of brevity, can we go with just `foldable`? Or `memFoldable`?

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