Implement -fsanitize=signed-integer-overflow.

gcc/ChangeLog

gcc/asan.c

gcc/c-family/ChangeLog

gcc/c-family/c-gimplify.c

gcc/config/i386/i386.md

gcc/flag-types.h

gcc/fold-const.c

gcc/gimple-fold.c

gcc/internal-fn.c

gcc/internal-fn.def

gcc/optabs.def

gcc/opts.c

gcc/predict.c

gcc/predict.h

gcc/sanitizer.def

gcc/testsuite/ChangeLog

gcc/testsuite/c-c++-common/ubsan/overflow-add-1.c

gcc/testsuite/c-c++-common/ubsan/overflow-add-2.c

gcc/testsuite/c-c++-common/ubsan/overflow-mul-1.c

gcc/testsuite/c-c++-common/ubsan/overflow-mul-2.c