graphite: Add runtime alias checking

graphite: Add runtime alias checking

Graphite rejects a SCoP if it contains a pair of data references for
which it cannot determine statically if they may alias. This happens
very often, for instance in C code which does not use explicit
"restrict".  This commit adds the possibility to analyze a SCoP
nevertheless and perform an alias check at runtime.  Then, if aliasing
is detected, the execution will fall back to the unoptimized SCoP.

TODO This needs more testing on non-OpenACC code.

gcc/ChangeLog:

        * common.opt: Add fgraphite-runtime-alias-checks.
        * graphite-isl-ast-to-gimple.cc
        (generate_alias_cond): New function.
        (graphite_regenerate_ast_isl): Use from here.
        * graphite-poly.cc (new_scop): Create unhandled_alias_ddrs vec ...
	(free_scop): and release here.
        * graphite-scop-detection.cc (dr_defs_outside_region): New function.
        (dr_well_analyzed_for_runtime_alias_check_p): New function.
        (graphite_runtime_alias_check_p): New function.
        (build_alias_set): Record unhandled alias ddrs for later alias check
        creation if flag_graphite_runtime_alias_checks is true instead
        of failing.
        * graphite.h (struct scop): Add field unhandled_alias_ddrs.
        * sese.h (has_operands_from_region_p): New function.

gcc/testsuite/ChangeLog:

        * gcc.dg/graphite/alias-1.c: New test.