c-family: Fix PR94272 -fcompare-debug issue even for C [PR99230]

c-family: Fix PR94272 -fcompare-debug issue even for C [PR99230]

The following testcase results in -fcompare-debug failure.
The problem is the similar like in PR94272
https://gcc.gnu.org/pipermail/gcc-patches/2020-March/542562.html
When genericizing, with -g0 we have just a TREE_SIDE_EFFECTS DO_STMT
in a branch of if, while with -g we have that wrapped into
TREE_SIDE_EFFECTS STATEMENT_LIST containing DEBUG_BEGIN_STMT and that
DO_STMT.
The do loop is empty with 0 condition, so c_genericize_control_stmt
turns it into an empty statement (without TREE_SIDE_EFFECTS).
For -g0 that means that suddenly the if branch doesn't have side effects
and is expanded differently.  But with -g we still have TREE_SIDE_EFFECTS
STATEMENT_LIST containing DEBUG_BEGIN_STMT and non-TREE_SIDE_EFFECTS stmt.
The following patch fixes that by detecting this case and removing
TREE_SIDE_EFFECTS.

And, so that we don't duplicate the same code, changes the C++ FE to
just call the c_genericize_control_stmt function that can now handle it.

2021-03-20  Jakub Jelinek  <jakub@redhat.com>

	PR debug/99230
	* c-gimplify.c (c_genericize_control_stmt): Handle STATEMENT_LIST.

	* cp-gimplify.c (cp_genericize_r) <case STATEMENT_LIST>: Remove
	special code, instead call c_genericize_control_stmt.

	* gcc.dg/pr99230.c: New test.