glsl/nir: Use nir_type_conversion_op
glsl/nir: Use nir_type_conversion_op

Using the helper is way better than hand-coding the universe.

Reviewed-by: Eric Anholt <96f164ad4d9b2b0dacf8ebee2bb1eeb3aa69adf1@anholt.net>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler