c750ab8a38
Check the bytemask against 0xFFFF rather than 0xF so that the fill is
skipped for a .xyzw write rather than a .x write.
Set the mask on the store to 0xF when doing a read so that all
components are written back.
Fixes: 31d26ebf1b ("pan/mdg: Fill from TLS before spilling non-SSA nodes")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16382>