Optimize if-else blocks

[anton-k]

Right now all expressions that go into if-else blocks are executed.
For all branches. It's better to place the expressions in separate branches:

It should be:

if (cond) {
  a1 = b1
  res = a1
} else {
  a2 = b2
  res = a2
}

But right now it's

a1 = b1
a2 = b2
if (cond) {
  res = a1
} else {  
  res = a2
}