[globalisel][tablegen] Demote OptForSize/OptForMinSize/ForCodeSize to per-function predicates.

Summary:
This causes them to be re-computed more often than necessary but resolves
objections that were raised post-commit on r301750.

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

Reviewed By: qcolombet

Subscribers: igorb, llvm-commits

Differential Revision: https://reviews.llvm.org/D32861

llvm-svn: 303418