[patch, fortran] Clobber some intent(out) variables on call

[Thomas Koenig]

Hello world,

the attached patch lets the middle-end know that variables
associated with intent(out) arguments become undefined, by
issuing an assignment to a special value (a "clobber")
before entering the procedure.

Originally, I had also planned to do so on entry to the
procedure, see https://gcc.gnu.org/ml/fortran/2018-09/msg00148.html .
This turned out to cause regressions; some details are outlined
in the PR.

Regression-tested. OK for trunk?

Regards

	Thomas

2018-09-22  Thomas Koenig  <tkoenig@gcc.gnu.org>

         PR fortran/41453
         * trans.h (gfc_conv_expr_reference): Add optional argument
         add_clobber to prototype.
         (gfc_conv_procedure_call):  Set add_clobber argument to
         gfc_conv_procedure_reference to true for scalar, INTENT(OUT),
         non-pointer, non-allocatable, non-dummy variables whose type
         is neither BT_CHARACTER, BT_DERIVED or BT_CLASS, but only if
         the procedure is not elemental.
         * trans-expr.c (gfc_conv_procedure_reference): Add clobber
         statement before call if add_clobber is set.

2018-09-22  Thomas Koenig  <tkoenig@gcc.gnu.org>

         PR fortran/41453
         * gfortran.dg/intent_optimize_2.f90: New test.
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