pan/bi: Fix load_const of 1-bit booleans

For historical reasons, we ingest 1-bit booleans in NIR but expand them
to 16/32-bit booleans in the backend IR. We need to handle this case
when loading boolean constants, extending from 1-bit to 16/32-bit as
required. This issue is masked by effective constant folding for
booleans, but is visible in a shader from Firefox WebRender.

Fixes: 646e03c4 ("pan/bi: Temporarily switch back to 0/~0 bools")
Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reported-by: Icecream95
Closes: #5797
Part-of: <!14371>
1 job for main in 11 minutes and 33 seconds (queued for 4 seconds)