[SDISel] Teach the type legalizer about ADDRSPACECAST (#90969)

Vectorized ADDRSPACECASTs were not supported by the type legalizer.

This patch adds the support for:
- splitting the vector result: <2 x ptr> => 2 x <1 x ptr>
- scalarization: <1 x ptr> => ptr
- widening: <3 x ptr> => <4 x ptr>

This is all exercised by the added NVPTX tests.