nir: Convert the if-test for num_inputs == 2 to an assertion
nir: Convert the if-test for num_inputs == 2 to an assertion

Suggested by Jason on a different patch after some comments /
questions by Ilia.

Signed-off-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>
Reviewed-by: Jason Ekstrand <b186559eaedf698788888c8a2559b4efb4db10d6@intel.com>
Reviewed-by: Connor Abbott <0619d315676a586e1ac7ebb4b6ada3f7c1df671a@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler