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

[Matthias Braun via llvm-commits]

MatzeB wrote:

Calling a `LiveInterval` "spillable" makes sense to me to, but not a machine operand. Also consider that we can very well spill most of the registers even if this bit is not set on the operands! (We just cannot fold the reload/spills into the instruction)

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.

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