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.
Use direct types in Sparc def : Pat patterns.

The SelectionDAG graph has MVT type labels, not register classes, so
this makes it clearer what is happening.

This notation is also robust against adding more types to the IntRegs
register class.