ac/llvm: replace structured by vindex != NULL in ac_build_buffer_store_common

ac/llvm: replace structured by vindex != NULL in ac_build_buffer_store_common

"raw" (IDXEN=0) and "structured" (IDXEN=1) do bounds checking differently.
From `si_make_buffer_descriptor`:
    * - For VMEM and inst.IDXEN == 0 or STRIDE == 0, it's in byte units.
    * - For VMEM and inst.IDXEN == 1 and STRIDE != 0, it's in units of STRIDE.

so there is a difference between setting vindex = i32_0 and vindex = NULL.
Instead of having the `structured` flag, we can just check if vindex is NULL.

Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15098>