nir/lower_memcpy: Don't mask the store

For constant-size memcpys, we can do as much as a vec4 at a time.  We
were accidentally masking the store to only the .x component.

Fixes: a3177cca996145 "nir: Add a lowering pass to lower memcpy"
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7305>
1 file changed