[TableGen] Fix some mismatches in the use of Namespace fields versus Target...

[TableGen] Fix some mismatches in the use of Namespace fields versus Target name in some of our emitters.

Some of our emitters were using the name of the Target to reference things that were created by others emitters using Namespace.

Apparently all targets have the same Target name as their instruction and register Namespace field?

Someone on IRC had a target that didn't do this and was getting build errors. This patch is a necessary, but maybe not sufficient fix.

llvm-svn: 307358