Generalize MVT::ValueType and associated functions to be able to represent

Generalize MVT::ValueType and associated functions to be able to represent
extended vector types. Remove the special SDNode opcodes used for pre-legalize
vector operations, and the special MVT::Vector type used with them. Adjust
lowering and legalize to work with the normal SDNode kinds instead, and to
use the normal MVT functions to work with vector types instead of using the
two special operands that the pre-legalize nodes held.

This allows pre-legalize and post-legalize DAGs, and the code that operates
on them, to be more consistent. Pre-legalize vector operators can be handled
more consistently with scalar operators. And, -view-dag-combine1-dags and
-view-legalize-dags now look prettier for vector code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@37719 91177308-0d34-0410-b5e6-96231b3b80d8