fortran: Support optional dummy as BACK argument of MINLOC/MAXLOC.

Protect the evaluation of BACK with a check that the reference is non-null
in case the expression is an optional dummy, in the inline code generated
for MINLOC and MAXLOC.

This change contains a revert of the non-testsuite part of commit
r15-1994-ga55d24b3cf7f4d07492bb8e6fcee557175b47ea3, which factored the
evaluation of BACK out of the loop using the scalarizer.  It was a bad idea,
because delegating the argument evaluation to the scalarizer makes it
cumbersome to add a null pointer check next to the evaluation.

Instead, evaluate BACK at the beginning, before scalarization, add a check
that the argument is present if necessary, and evaluate the resulting
expression to a variable, before using the variable in the inline code.

gcc/fortran/ChangeLog:

	* trans-intrinsic.cc (maybe_absent_optional_variable): New function.
	(gfc_conv_intrinsic_minmaxloc): Remove BACK from scalarization and
	evaluate it before.  Add a check that BACK is not null if the
	expression is an optional dummy.  Save the resulting expression to a
	variable.  Use the variable in the generated inline code.

gcc/testsuite/ChangeLog:

	* gfortran.dg/maxloc_6.f90: New test.
	* gfortran.dg/minloc_7.f90: New test.

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