[NVPTX] Failure to lower `v2i8`

[justinfargnoli]

Lowering v2i8 as a return type is causing a compiler crash.

Reproducer on compiler explorer: https://godbolt.org/z/znxzW9Eqj

Originally posted by @Artem-B in #104524 (comment)

[justinfargnoli]

Reproducer:

define <2 x i8> @byte2() {
  ret <2 x i8> zeroinitializer
}

[justinfargnoli]

This is caused by SelectionDAG wanting to lower v2i8 as 1 v2i16 value while ComputePTXValueVTs wants to lower it to 2 i8 values.

[justinfargnoli]

@manasij7479 is going to look into this.