[NVPTX] fix emission for i1 load and extload (#99392)
Currently, an illegal 2-byte load from a 1-byte global variable is being
generated. This change instead generates a 1-byte load and zero-extends
it to i16 register. This was always the intended behavior of the
function.
In addition, an i1 ext load of any kind needs to be promoted. A missing
setLoadExtAction for ISD::EXTLOAD was causing an "Unhandled source type"
unreachable due to an illegal i1 ext load during ISelDAGtoDAG (see below
bug).
Bug https://github.com/llvm/llvm-project/issues/98033.
