[TableGen] Make CodeGenMapTable understand the namespace field of an instruction

Do not force the backends to use target name as namespace.

Original patch by Mattias Eriksson

Reviewers: stoklund, craig.topper

Reviewed By: stoklund

Subscribers: materi, llvm-commits

Differential Revision: https://reviews.llvm.org/D31322

llvm-svn: 298834