nir/spirv: Rework decoration iteration
nir/spirv: Rework decoration iteration

The old code didn't work correctly if you had member decorations after
non-member decorations.  Since glslang never gave us any of those, it
wasn't properly tested.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler