glsl: fix key used for hashing switch statement cases

glsl: fix key used for hashing switch statement cases

Implementation previously used value itself as the key, however after
hash implementation change by ee02a5e we cannot use 0 as key.

v2: use constant pointer as the key and implement comparison
    for contents (Eric Anholt)

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97309