[globalisel][tablegen] Add control-flow to the MatchTable.

[globalisel][tablegen] Add control-flow to the MatchTable.

Summary:
This will allow us to merge the various sub-tables into a single table. This is a
compile-time saving at this point. However, this will also enable the optimization
of a table so that similar instructions can be tested together, reducing the time
spent on the matching the code.

The bulk of this patch is a mechanical conversion to the new MatchTable object
which is responsible for tracking label definitions and filling in the index of
the jump targets. It is also responsible for nicely formatting the table.

This was necessary to support the new GIM_Try opcode which takes the index to
jump to if the match should fail. This value is unknown during table
construction and is filled in during emission. To support nesting try-blocks
(although we currently don't emit tables with nested try-blocks), GIM_Reject
has been re-introduced to explicitly exit a try-block or fail the overall match
if there are no active try-blocks.

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

Reviewed By: rovka

Subscribers: kristof.beyls, igorb, llvm-commits

Differential Revision: https://reviews.llvm.org/D35117

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@308596 91177308-0d34-0410-b5e6-96231b3b80d8