[globalisel][tablegen] Require that all registers between instructions of a match are virtual.

Summary:
Without this, it's possible to encounter multiple defs for a register.

This is triggered by the current version of D32868 when applied to trunk.

Reviewers: qcolombet, ab, t.p.northover, rovka, kristof.beyls

Reviewed By: qcolombet

Subscribers: llvm-commits, igorb

Differential Revision: https://reviews.llvm.org/D32869

llvm-svn: 303253