TableGen: Change { } to only accept bits<n> entries when n == 1.

Prior to this change, it was legal to do something like

  bits<2> opc = { 0, 1 };
  bits<2> opc2 = { 1, 0 };
  bits<2> a = { opc, opc2 };

This involved silently dropping bits from opc and opc2 which is very hard to debug.

Now the above test would be an error.  Having tested with an assert, none of LLVM/clang was relying on this behaviour.

Thanks to Adam Nemet for the above test.

llvm-svn: 215083