[GlobalISel][InstructionSelect] Refactoring buildMatchTable out, NFC

to share it between the Instruction Selector in optimized and
non-optimized modes both and the Testgen.

Reviewers: dsanders, aemerson

Reviewed By: dsanders

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

Differential Revision: https://reviews.llvm.org/D46097

llvm-svn: 331396