intel/compiler: fix alignment assert in nir_emit_intrinsic
Fixes: c6439792 (intel/fs: Choose memory message type based on bit size)
This assert was introduced in !4367 (merged) and is triggered by the Vulkan CTS test dEQP-VK.subgroups.ballot_broadcast.compute.subgroupbroadcast_i8vec2 (and probably others like it)