bf29040103
The previous fix worked when the second channel wasn't exposed, but
a couple of piglit tests have inputs with just the y/z chans, no x/w.
Partly Fixes piglit ext_transform_feedback-immediate-reuse-index-buffer
with llvmpipe/nir
Fixes: 5363cda52b ("gallivm: add swizzle support where one channel isn't defined.")
Reviewed-by: Roland Scheidegger <sroland@vmware.com>