feat(NodePath.prune): deal with an IfStatement not having its consequent...

... after a prune.

If an IfStatement has it's consequent pruned and there is no alternate leave the test as
a ExpressionStatement. If the alternate still exists negate the test and move the alternate
to the consequent position.
My convention is to declare `var b = types.builders;` at the top of the file (just after `var n = types.namedTypes;`) so you can save typing here: `b.expressionStatement(...)`.