intel/nir: Limit unaligned loads to vec4

intel/nir: Limit unaligned loads to vec4

This probably doesn't affect Vulkan or GL because they can't have
anything bigger than a vec4 anyway unless it's a u64vec4 and those have
to be at least 8B aligned.  This may affect CL apps if they use
__attribute__((packed)) on something with big vectors, depending on how
LLVM decides to translate that.

Fixes: f8aa83f0c86e ("intel/nir: Use nir_lower_mem_access_bit_sizes()")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21524>