Allow direct value types in pattern definitions.

Just like register classes, value types can be used in two ways in
patterns:

  (sext_inreg i32:$src, i16)

In a named leaf node like i32:$src, the value type simply provides the
type of the node directly. This simplifies type inference a lot compared
to the current practice of specifiying types indirectly with register
classes.

As an unnamed leaf node, like i16 above, the value type represents
itself as an MVT::Other immediate.

llvm-svn: 177828