[X86InstrInfo] support memfold on spillable inline asm (#70832)

This enables -regalloc=greedy to memfold spillable inline asm
MachineOperands.

Because no instruction selection framework marks MachineOperands as
spillable, no language frontend can observe functional changes from this
patch. That will change once instruction selection frameworks are
updated.

Link: https://github.com/llvm/llvm-project/issues/20571