nir: Silence missing field initializer warnings for vectors in nir_constant_expressions

nir: Silence missing field initializer warnings for vectors in nir_constant_expressions

nir/nir_constant_expressions.c: In function 'evaluate_ball2':
nir/nir_constant_expressions.c:279:7: warning: missing initializer for field 'z' of 'struct bool_vec' [-Wmissing-field-initializers]
       };
       ^
nir/nir_constant_expressions.c:234:10: note: 'z' declared here
    bool z;
          ^

Number of total warnings in my build reduced from 2532 to 2304
(reduction of 228).

v2: Initialize bool vectors with 0 instead of false to keep the
generator simpler.  Suggested by Ken.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>