Branch culling of dead if statement branches is not consistent

[rikkimax]

The following code works in full D, but not in -betterC.

void func() nothrow {
    if (0) {
     	 throw new Exception("");  // Error: cannot use `throw` statements with -betterC
    }
}

This ticket is not necessarily for this exact snippet, but may be implying issues with dmd's glue code, not replicating the logic in blockexit.d for if statements.