ac/llvm: Remove "structurized" argument and instead check vindex.

ac/llvm: Remove "structurized" argument and instead check vindex.

Change ac_build_buffer_load_common and ac_build_tbuffer_load so
the use structurized load when the vindex argument is not NULL.
Adjust callers to match the new behaviour.

This fixes the load_buffer_amd intrinsic with index source.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Qiang Yu <yuq825@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Acked-by: Konstantin Seurer <konstantin.seurer@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21358>