glsl: Generate strings that are the enum names without the ir_*op_ prefix

glsl: Generate strings that are the enum names without the ir_*op_ prefix

For many expressions, this is different from the printable name.  The
printable name for ir_binop_add is "+", but we want "add".  This is
needed for ir_builder_print_visitor.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>