GlobalISel: forbid physical registers on generic MIs.

GlobalISel: forbid physical registers on generic MIs.

We're intending to move to a world where the type of a register is determined
by its (unique) def. This is incompatible with physregs, which are untyped.

It also means the other passes don't have to worry quite so much about
register-class compatibility and inserting COPYs appropriately.

llvm-svn: 280132