nir/lower_explicit_io: Handle 1 bit loads and stores

Load a 32-bit value then convert to 1-bit.  Convert 1-bit to 32-bit
value, then Store it.

These cases started to appear when we changed Anvil to use derefs for
shared memory.

v2: Use `bit_size` in a couple of places we were missing.  (Jason)
    Reassign `value` instead of `src[0]`.  (Jason)

Fixes: 024a46a4 ("anv: use derefs for shared memory access")
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit c0c55bd8)