[MachineOutliner] Freeze registers in new functions

Summary:
Call MRI.freezeReservedRegs() on functions created during outlining so
that calls to isReserved() by the verifier called after this pass won't
assert.

Reviewers: MatzeB, qcolombet, paquette

Subscribers: mcrosier, javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D42749

llvm-svn: 323905