[GlobalISel][InstructionSelect] Making Coverage Info generation optional on per-match table basis

to make sure that Testgen always has access to coverage info even if
the match table used by the selector itself is stripped off that
information for performance reasons.

Reviewers: dsanders, aemerson

Reviewed By: dsanders

Subscribers: rovka, kristof.beyls, llvm-commits, dsanders

Differential Revision: https://reviews.llvm.org/D46098

llvm-svn: 331398