[globalisel][tablegen] Add support for EXTRACT_SUBREG.

Summary:
After this patch, we finally have test cases that require multiple
instruction emission.

Depends on D33590

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

Subscribers: javed.absar, llvm-commits, igorb

Differential Revision: https://reviews.llvm.org/D33596

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@306388 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed