[patch, fortran] Fix PR 85544

[Thomas Koenig]

Hello world,

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?

Regards

	Thomas

2018-12-16  Thomas Koenig  <tkoenig@gcc.gnu.org>

         PR fortran/85544
         * frontend-passes.c (optimize_power): Remove.
         (optimize_op): Remove call to optimize_power.
         * trans-expr.c (gfc_conv_power_op): Handle cases of 1**integer,
         (2|4|8|16) ** integer and (-1) ** integer.

2018-12-16  Thomas Koenig  <tkoenig@gcc.gnu.org>

         PR fortran/85544
         * gfortran.dg/power_7.f90: New test.
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