[globalisel] Stop requiring -debug/-debug-only=registerbankinfo for assertions.

Summary:
I've noticed that these assertions don't trigger when the condition is false.
The problem is that the DEBUG(x) macro only executes x when the pass is
emitting debug output via the -debug and -debug-only=registerbankinfo command
line arguments.

Debug builds should always execute the assertions so use '#ifndef NDEBUG' instead.

Also removed an assertion that is only true the first time it's tested. <Target>RegisterBankInfo's constructor will re-use register banks causing them to be valid on subsequent tests. That
assertion will fail on the first test too in the near future.

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

Subscribers: dberris, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D28358

llvm-svn: 291235