ubsan: missed -fsanitize=bounds for compound ops [PR108060]

ubsan: missed -fsanitize=bounds for compound ops [PR108060]

In this PR we are dealing with a missing .UBSAN_BOUNDS, so the
out-of-bounds access in the test makes the program crash before
a UBSan diagnostic was emitted.  In C and C++, c_genericize gets

  a[b] = a[b] | c;

but in C, both a[b] are one identical shared tree (not in C++ because
cp_fold/ARRAY_REF created two same but not identical trees).  Since
ubsan_walk_array_refs_r keeps a pset, in C we produce

  a[.UBSAN_BOUNDS (0B, SAVE_EXPR <b>, 8);, SAVE_EXPR <b>;] = a[b] | c;

because the LHS is walked before the RHS.

Since r7-1900, we gimplify the RHS before the LHS.  So the statement above
gets gimplified into

    _1 = a[b];
    c.0_2 = c;
    b.1 = b;
    .UBSAN_BOUNDS (0B, b.1, 8);

With this patch we produce:

  a[b] = a[.UBSAN_BOUNDS (0B, SAVE_EXPR <b>, 8);, SAVE_EXPR <b>;] | c;

which gets gimplified into:

    b.0 = b;
    .UBSAN_BOUNDS (0B, b.0, 8);
    _1 = a[b.0];

therefore we emit a runtime error before making the bad array access.

I think it's OK that only the RHS gets a .UBSAN_BOUNDS, as in few lines
above: the instrumented array access dominates the array access on the
LHS, and I've verified that

  b = 0;
  a[b] = (a[b], b = -32768, a[0] | c);

works as expected: the inner a[b] is OK but we do emit an error for the
a[b] on the LHS.

For GCC 14, we could apply
<https://gcc.gnu.org/pipermail/gcc-patches/2023-March/613687.html>
since the copy_node doesn't seem to be needed.

	PR sanitizer/108060
	PR sanitizer/109050

gcc/c-family/ChangeLog:

	* c-gimplify.cc (ubsan_walk_array_refs_r): For a MODIFY_EXPR, instrument
	the RHS before the LHS.

gcc/testsuite/ChangeLog:

	* c-c++-common/ubsan/bounds-17.c: New test.
	* c-c++-common/ubsan/bounds-18.c: New test.
	* c-c++-common/ubsan/bounds-19.c: New test.
	* c-c++-common/ubsan/bounds-20.c: New test.
	* c-c++-common/ubsan/bounds-21.c: New test.