nir/spirv/glsl450: Add an 'nb' shortcut variable.
nir/spirv/glsl450: Add an 'nb' shortcut variable.

"nb" is shorter and more convenient than "&b->nb", especially
when several operations are composed together into a larger expression
tree.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler