TableGen: Refactor AsmWriterEmitter to keep AsmWriterInsts.

These used to be referenced by the CGI->AWI map (in AsmWriterEmitter), but
stored in a vector local to EmitPrintInstruction. Move the vector to
AsmWriterEmitter too.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193525 91177308-0d34-0410-b5e6-96231b3b80d8
TableGen: Refactor DAG patterns to enable parsing one pattern at a time.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193526 91177308-0d34-0410-b5e6-96231b3b80d8