Proper C scopes for if, try and case statements

even thought the setjmp implementation for try statement won't be used in C++,
using properly scoped variables for them is beneficial, because we'll be able to
establish a 1:1 relation between nimrod scopes and C scopes.
Once we have that, we'll be able to keep the original names of local variables
to greatly improve the debugging experience (i.e. watch expressions and
hover tooltips will work).

compiler/cgendata.nim

@@ -65,6 +65,8 @@ type
                               # before 'break'|'return'
     labels*: Natural          # for generating unique labels in the C proc
     blocks*: seq[TBlock]      # nested blocks
+    breakIdx*: int            # the block that will be exited
+                              # with a regular break
     options*: TOptions        # options that should be used for code
                               # generation; this is the same as prc.options
                               # unless prc == nil