[globalisel][tablegen] Implement unindexed load, non-extending load, and MemVT checks

Summary:
This includes some context-sensitivity in the MVT to LLT conversion so that
pointer types are tested correctly.
FIXME: I'm not happy with the way this is done since everything is a
       special-case. I've yet to find a reasonable way to implement it.

select-load.mir fails because <1 x s64> loads in tablegen get priority over s64
loads. This is fixed in the next patch and as such they should be committed
together, I've posted them separately to help with the review.

Depends on D37456

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

Subscribers: kristof.beyls, javed.absar, llvm-commits, igorb

Differential Revision: https://reviews.llvm.org/D37457

llvm-svn: 315884