ir_minimizer_main should be able to do simple x-proc return value simplifications

the minimizer cannot change the return type of subroutines (eg remove tuple elements). This can somewhat reduce the effectiveness of the minimization by leaving extra instructions to fill in these unneeded fields if the minimizer is unable to inline the invoke away.

It would be nice to support this by replacing uses of those fields in the caller with literals which can later be reduced away.