Downgrade a tablegen warning to an error.
Downgrade a tablegen warning to an error.

Ambiguous sub-register index compositions are OK as long as the backend writer
knows what he is doing.

git-svn-id: a4a6f32337ebd29ad4763b423022f00f68d1c7b7@131134 91177308-0d34-0410-b5e6-96231b3b80d8

lodyagin/bare_cxx,lodyagin/bare_cxx,lodyagin/bare_cxx,lodyagin/bare_cxx,lodyagin/bare_cxx