llvmpipe/draw: handle constant buffer limits and robustness (v1.1)

llvmpipe/draw: handle constant buffer limits and robustness (v1.1)

TGSI expect vec4 of constants for it's current code paths, and when
doing indirect accesses it does the comparison on vec4 indexes,
however NIR does the indexing on packed float indexes.

This also align the compute path with the other shaders, and
should improve robustness (at least under Vulkan)

Fixes:
KHR-NoContext.gl43.robust_buffer_access_behavior.uniform_buffer

v1.1:
rename variable to something more meaningful (Roland)

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5971>