[Statepoint Lowering] Add an option to allow use gc values in regs for landing pad

[skatkov]

Summary

Default value is not changed, so it is NFC actually.

The option allows to use gc values on registers in landing pads.

[dantrushin]

LGTM.
Though, I don't see much point in having it :)

[reames]

LGTM as a testing aid. Should be removed once invoke support is stabilized and fully enabled.