glsl: Fix tautological comparison.
glsl: Fix tautological comparison.

Caught by clang.

warning: comparison of constant -1 with expression of type
         'ir_texture_opcode' is always false
      [-Wtautological-constant-out-of-range-compare]
      if (op == -1)
          ~~ ^  ~~

Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler