nir: Fix unaligned pointer access

This was observed with the intel vulkan driver when running
dEQP-VK.spirv_assembly.instruction.compute.float32.comparison_1.modfstruct
with ubsan enabled.

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6728>
1 file changed