openacc: Add data optimization pass

openacc: Add data optimization pass

Address PR90591 "Avoid unnecessary data transfer out of OMP
construct", for simple (but common) cases.

This commit adds a pass that optimizes data mapping clauses.
Currently, it can optimize copy/map(tofrom) clauses involving scalars
to copyin/map(to) and further to "private".  The pass is restricted
"kernels" regions but could be extended to other types of regions.

gcc/ChangeLog:

        * Makefile.in: Add pass.
        * doc/gimple.texi: TODO.
        * gimple-walk.cc (walk_gimple_seq_mod): Adjust for backward walking.
        * gimple-walk.h (struct walk_stmt_info): Add field.
        * passes.def: Add new pass.
        * tree-pass.h (make_pass_omp_data_optimize): New declaration.
        * omp-data-optimize.cc: New file.

libgomp/ChangeLog:

        * testsuite/libgomp.oacc-c-c++-common/kernels-decompose-1.c:
	Expect optimization messages.
        * testsuite/libgomp.oacc-fortran/pr94358-1.f90: Likewise.

gcc/testsuite/ChangeLog:

        * c-c++-common/goacc/note-parallelism-1-kernels-loops.c: Likewise.
	* c-c++-common/goacc/note-parallelism-1-kernels-straight-line.c:
	Likewise.
	* c-c++-common/goacc/note-parallelism-kernels-loops.c: Likewise.
        * c-c++-common/goacc/uninit-copy-clause.c: Likewise.
        * gfortran.dg/goacc/uninit-copy-clause.f95: Likewise.
        * c-c++-common/goacc/omp_data_optimize-1.c: New test.
        * g++.dg/goacc/omp_data_optimize-1.C: New test.
        * gfortran.dg/goacc/omp_data_optimize-1.f90: New test.

Co-Authored-By: Thomas Schwinge <thomas@codesourcery.com>