[GlobalISel][InstructionSelect] Refactoring out a getMatchTable virtual method + other small NFC's

The main goal is to share getMatchTable between the Instruction
Selector and the Testgen.

The commit also contains some NFC only loosely related to refactoring
out the getMatchTable, but strongly related to the initial Testgen
patch (see https://reviews.llvm.org/D43962)

Reviewers: dsanders, aemerson

Reviewed By: dsanders

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

Differential Revision: https://reviews.llvm.org/D46096

llvm-svn: 331395