[NVPTX] Fixup v2i8 parameter and return lowering (#145585)
This change fixes v2i8 lowering for parameters and returned values. As
part of this work, I move the lowering for return values to use generic
ISD::STORE nodes as these are more flexible and have existing
legalization handling.
Note that calling a function with v2i8 arguments or returns is still not
working but this is left for a subsequent change as this MR is already
fairly large.
Partially addresses #128853
