[globalisel][tablegen] Add support for importing 'imm' operands.

[globalisel][tablegen] Add support for importing 'imm' operands.

Summary:
This patch enables the import of rules containing 'imm' operands that do not
constrain the acceptable values using predicates. Support for ImmLeaf will
arrive in a later patch.

Depends on D35681

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

Reviewed By: rovka

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

Differential Revision: https://reviews.llvm.org/D35833

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310343 91177308-0d34-0410-b5e6-96231b3b80d8