nir_to_tgsi: Use nir_opt_offsets for load_ubo_vec4.

nir_to_tgsi: Use nir_opt_offsets for load_ubo_vec4.

This helps non-native-integers hardware where relative addressing of UBOs
has a constant offset field, and having addressing math (particularly for
D3D9) emitted as ALU ops ends up running us out of constants.  For
native-integers drivers (such as softpipe), the possible-overflow check
typically triggers and we end up not folding.

r300:
total instructions in shared programs: 1279167 -> 1278731 (-0.03%)
instructions in affected programs: 50834 -> 50398 (-0.86%)
total temps in shared programs: 213736 -> 213687 (-0.02%)
temps in affected programs: 598 -> 549 (-8.19%)
total consts in shared programs: 952973 -> 952850 (-0.01%)
consts in affected programs: 26776 -> 26653 (-0.46%)

Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14309>