[IR] Fix ReceiveExceptionInstr.cloneForInlining
[IR] Minor code cleanup.

* toStringVariables is not really useful since all scopes
  have their CFG built always.
[IR] Bug fixes handling returns in the presence of ensure blocks

* In IR mode, the following code would print '2' instead of '1'
  which is the expectation.

def foo
  v = 1
  begin
    return v
  ensure
    v = 2
  end
end

p foo

* Fixed IR building (and OptimizeTempVarsPass) to implement this
  behavior.

* Overhaul of ensure block IR is coming up in coming days that will
  eliminate some of this complexity.

* No change in rubyspec run (we are missing a rubyspec for this)

* TODO: Add new rubyspec test