Add MachineVerifier check for AllVRegsAllocated MachineFunctionProperty

Summary:
Check that any function that has the property set is free of virtual
register operands.

Also, it is actually VirtRegMap (and not the register allocators) that
acutally remove the VReg operands (except for RegAllocFast).

Reviewers: qcolombet

Subscribers: MatzeB, llvm-commits, qcolombet

Differential Revision: http://reviews.llvm.org/D18535

llvm-svn: 264755