nir: Length of boolean vtn_value now is 1

nir: Length of boolean vtn_value now is 1

During conversion type-length was lost due to math.

v2 (Jason Ekstrand):
 - Use a size/offset of 4 bytes

Fixes: 44227453ec03 (nir: Switch to using 1-bit Booleans for almost everything)
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109353
Signed-off-by: Sergii Romantsov <sergii.romantsov@globallogic.com>
Tested-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>