Change *EXTLOAD to use an VTSDNode operand instead of being an MVTSDNode.

Change *EXTLOAD to use an VTSDNode operand instead of being an MVTSDNode.
This is the last MVTSDNode.

This allows us to eliminate a bunch of special case code for handling
MVTSDNodes.

Also, remove some uses of dyn_cast that should really be cast (which is
cheaper in a release build).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@22368 91177308-0d34-0410-b5e6-96231b3b80d8