[patch, fortran] Simplify (-1.0)**i

[Thomas Koenig]

Hello world,

the attached patch replaces (-1.0)**i with (in C language)
(i & 1) == 0 ? 1.0 : 1.0, see PR 57073.

I tried doing it in the middle end, see the PR of where these
approaches failed.  So, rather than not doing the optimization
at all, I would rather do it in the Fortran front end.

If somebody jumps in with a middle-end solution that works, I
would withdraw this patch.

Regression-tested on trunk.  OK?

	Thomas

2013-05-19  Thomas Koenig  <tkoenig@gcc.gnu.org>

         PR fortran/57073
         * trans-expr.c:  Simplify (-1.0)**i to (i & 1) == 0 ? 1.0 : -1.0.

2013-05-19  Thomas Koenig  <tkoenig@gcc.gnu.org>

         PR fortran/57073
         * gfortran.dg/power_6.f90:  New test.
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