[MIR] Reset unique MBB numbering in MachineFunction::reset()

No need to waste space nor number MBBs differently if MF gets recreated.

Reviewers: qcolombet, stoklund, t.p.northover, bogner, javed.absar

Reviewed By: qcolombet

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D46078

llvm-svn: 331213