Add support for Statepoint operand folding (replacing VReg with
memory operand). This allowed for InlineSpiller only, because it
knows how to reload tied def VReg after it was turned into memory
operand. Other callers (e.g. peephole) cannot do such reload.
We do this by un-tieing operand we want to fold in InlineSpiller
and allowing to fold only untied operands.
Full change set is available at D81603.