[globalisel][tablegen] Import ComplexPattern when used as an operator

Summary:
It's possible for a ComplexPattern to be used as an operator in a match
pattern. This is used by the load/store patterns in AArch64 to name the
suboperands returned by ComplexPattern predicate so that they can be broken
apart and referenced independently in the result pattern.

This patch adds support for this in order to enable the import of load/store
patterns.

Depends on D37445

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

Reviewed By: qcolombet

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

Differential Revision: https://reviews.llvm.org/D37456

llvm-svn: 315863