nir: Fix setup of constant bool initializers.

nir: Fix setup of constant bool initializers.

brw_fs_nir has only seen scalar bools so far, thanks to vector splitting,
and the ralloc of in glsl_to_nir.cpp will *usually* get you a 0-filled
chunk of memory, so reading too large of a value will usually get you the
right bool value.  But once we start doing vector bools in a few commits,
we end up getting bad values.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>