[GlobalISel] Add basic Selector-emitter tblgen backend.

[GlobalISel] Add basic Selector-emitter tblgen backend.

This adds a basic tablegen backend that analyzes the SelectionDAG
patterns to find simple ones that are eligible for GlobalISel-emission.

That's similar to FastISel, with one notable difference: we're not fed
ISD opcodes, so we need to map the SDNode operators to generic opcodes.
That's done using GINodeEquiv in TargetGlobalISel.td.

Otherwise, this is mostly boilerplate, and lots of filtering of any kind
of "complicated" pattern. On AArch64, this is sufficient to match G_ADD
up to s64 (to ADDWrr/ADDXrr) and G_BR (to B).

Differential Revision: https://reviews.llvm.org/D26878

llvm-svn: 290284