glsl: Refactor get_num_operands.

glsl: Refactor get_num_operands.

This adds sentinel values to the ir_expression_operation enum type:
ir_last_unop, ir_last_binop, and ir_last_opcode.  They are set to the
previous one so they don't trigger "unhandled case in switch statement"
warnings, but should never be handled directly.

This allows us to remove the huge array of 1s and 2s in
ir_expression::get_num_operands().