Eliminate dead code for clj-nil and types that don't admit falsey

If the test of an if has a tag of clj-nil then the else branch will always be taken so we can replace such code with the else branch.

Likewise if the test of an if has a tag that cannot yield a falsey value (for example #{number string}), then the then branch will always be taken and we can replace such code with the then branch.

We already have dead code elimination for constants, this ticket asks that we extend it to cases where we can definitely determine a branch will be taken based on inferred types.