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.
For extra credit, perhaps detect if the expression is already a negated expression and remove the existing `!` in that case, rather than double-negating?