bugpoint: new pass.

bugpoint: new pass.

A typical use of `bugpoint` would involve a script with a pass under
test, e.g.:

    flowmap -relax -optarea 100

and would be invoked as:

    bugpoint -yosys ./yosys -script flowmap.ys -clean -cells

This replaces the current design with the minimal design that still
crashes the `flowmap.ys` script.

`bugpoint` can also be used to perform generic design minimization
using `select`, e.g. the following script:

    select i:* %x t:$_MUX_ %i -assert-max 0

would remove all parts of the design except for an unbroken path from
an input to an output port that goes through exactly one $_MUX_ cell.
(The condition is inverted.)