[gfortran] Support INTEGER<->LOGICAL conversion (take 3)

[Tobias Schlüter]

Roger Sayle wrote:
> To address Tobias' concerns about possible adverse interactions with
> fortran's arithmetic if I've double checked that there's run-time
> testing of this construct in gfortran.fortran-torture/arithmeticif.f90
> and gfortran.dg/pr17229.f, and that both of these tests continue to
> pass with this patch.

I'm sorry that I didn't think of this yesterday and only focused on arithmetic
if, but there are more possible interactions with the IF statement.  Is this
extension intended to support something like
   integer i
   if (i) then
     dostuff
   end if
or
   logical l
   if (l) 100, 200, 300 ! label 100 is never jumped to
?

This won't work with the patch, as the matcher for IF expects a logical /
integer expression and rejects the code otherwise, whereas the implicit
conversions are inserted during resolution, i.e. much later in the process.

- Tobi