Fix conditional call `f() if cond;` and conditional tail-call `return f() if cond;` statements.

[Bananattack]

I recently merged in a pull request for some new tests from @lhsazevedo in #122. This closed pull request documents the behaviour of the tests, but all of these should generate passing code.

• Conditional calls should be equivalent to either a specialized conditional call instruction (if it exists), or a conditional branch instruction around a block that contains a call instruction.
• Conditional tail calls should be equivalent to conditional branches -- or failing a tail-call optimization, a conditional branch instruction around a block containing a call instruction followed by a return instruction.