spirv: Use the right bit-size for spec constant ops

Previously, we would always pull the bit size from the destination which
is wrong for opcodes like nir_ilt where the sources are variable-sized
but the destination is a fixed size.  We were getting lucky before
because nir_op_ilt returns a 32-bit value and basically everyone who
uses spec constants uses 32-bit ones.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
(cherry picked from commit 8fa70cfc)