[tablegen][globalisel] Trivial changes to reduce size of following patch. NFC.

Summary:
* Move namespace {
* Trivial: Typo
* RuleMatcher: Separate class and definition
* Trivial: const findNodeEquiv

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

Reviewed By: rovka

Subscribers: dberris, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D30531

llvm-svn: 297884