[globalisel] OperandPredicateMatcher's shouldn't need to generate the MachineOperand expr. NFC

Summary:
Each OperandPredicateMatcher shouldn't need to know how to generate the expression
to reference a MachineOperand. The OperandMatcher should provide it.

In addition to separating responsibilities, this also lays some groundwork for
decoupling source patterns from destination patterns to allow invented operands
or operands provided by GlobalISel's equivalent to the ComplexPattern<> class.

Depends on D29709

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

Reviewed By: ab

Subscribers: dberris, kristof.beyls, llvm-commits, igorb

Differential Revision: https://reviews.llvm.org/D29710

llvm-svn: 295668