[MachineVerifier][GlobalISel] Verifying generic extends and truncates

Making sure we don't truncate / extend pointers, don't try to change
vector topology or bitcast vectors to scalars or back, and most
importantly, don't extend to a smaller type or truncate to a large
one.

Reviewers: qcolombet t.p.northover aditya_nandakumar

Reviewed By: qcolombet

Subscribers: rovka, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D46490

llvm-svn: 331718