*ping* [patch, fortran] Fix PR 85544

[Thomas Koenig]

Ping?

> the PR pointed out an old regression because the front-end optimization
> pass was substituting 2**n with ishift(1,n), where n was an array.
> 
> Simply removing the optimization for that case would have been easy,
> but also introduced a performance regression.
> 
> So, for this, I moved the optimization to trans-*, where it makes more
> sense.
> 
> Regression-tested.  This turned up that one of our tests, mvbits_1.f90,
> depends on the behavior that 2**32 is zero.  This is certainly not
> guaranteed by the standard, but I chose to keep the behavior as not
> to introduce any changes in behavior.
> 
> This fixes a regression, so I would like to backport to all active
> branches if this if possible.
> 
> Oh yes, if anybody feels strongly that we should also optimize 32**n
> and powers of other powers to two, now is the time to speak up :-)
> 
> OK for affected branches?