Handle if and unless modifiers correctly

[amomchilov]

The current implementation from #31 and #203 will always produce parser::If nodes.

If if or unless are used as modifiers (example below), they should lower to parser::IfGuard and parser::UnlessGuard instead.

if p # `parser::If`
  "if body"
else
  "else body"
end

unless p # `parser::If`, with the blocks swapped
  "unless body"
else
  "else body"
end

"value" if p     # Should be `parser::IfGuard`,     but is `parser::If` today.
"value" unless p # Should be `parser::UnlessGuard`, but is `parser::If` today.

[st0012]

I'm not sure what changed, but I think IfGuard is only used in pattern matching now.

This script

if condition; end

"value" if condition

generates

Begin {
  stmts = [
    If {
      condition = Send {
        receiver = NULL
        method = <U condition>
        args = [
        ]
      }
      then_ = NULL
      else_ = NULL
    }
    If {
      condition = Send {
        receiver = NULL
        method = <U condition>
        args = [
        ]
      }
      then_ = String {
        val = <U value>
      }
      else_ = NULL
    }
  ]
}

Only If nodes are used.

I think currently only conditions in pattern matching cases generate IfGuard.

Like

case bar
in x if x == 1
  "one!"
end

generates

    CaseMatch {
      expr = Send {
        receiver = NULL
        method = <U bar>
        args = [
        ]
      }
      inBodies = [
        InPattern {
          pattern = MatchVar {
            name = <U x>
          }
          guard = IfGuard {
            condition = Send {
              receiver = LVar {
                name = <U x>
              }
              method = <U ==>
              args = [
                Integer {
                  val = "1"
                }
              ]
            }
          }
          body = String {
            val = <U one!>
          }
        }
      ]
      elseBody = NULL
    }