intel/vec4: Fix nir_op_b2[fi] with 64-bit result

intel/vec4: Fix nir_op_b2[fi] with 64-bit result

This is valid NIR but you can't actually hit this case today.  GLSL IR
doesn't have a bool to double opcode; it does f2d(b2f(x)).  In SPIR-V we
don't have any to/from bool conversion opcodes at all.  However, the
next commit will make us start generating it so we should be ready.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>