[Statepoint] Turn assert into check in foldPatchpoint.

Original D81646 had check for tied regs in foldPatchpoint().
Due to unfortunate miscommunication with review comments and
adressing some comments post commit, it turned into assertion.

We had an offline talk and agreed that with current implementation
this path is possible, so I'm changing it back to check.

Note that this is workaround until ussues described in PR46917 are
resolved.