[SelectionDAG] Remove special methods for creating *_EXTEND_VECTOR_INREG...

[SelectionDAG] Remove special methods for creating *_EXTEND_VECTOR_INREG nodes. Move asserts into getNode.

These methods were just wrappers around getNode with additional asserts (identical and repeated 3 times). But getNode already has a switch that can be used to hold these asserts that allows them to be shared for all 3 opcodes. This also enables checking on the places that create these nodes without using the wrappers.

The rest of the patch is just changing all callers to use getNode directly.

llvm-svn: 346087