c++: Fix a -fcompare-debug issue with DEBUG_BEGIN_STMT stmts in STATEMENT_LISTs ...

c++: Fix a -fcompare-debug issue with DEBUG_BEGIN_STMT stmts in STATEMENT_LISTs [PR94272]

The following testcase FAILs with -fcompare-debug.  The problem is that
the C++ FE initially uses IF_STMTs, tcc_statement which default to
TREE_SIDE_EFFECTS set, but later on is genericized into COND_EXPRs,
tcc_expression which default to TREE_SIDE_EFFECTS ored from all 3 operands.
Furthermore, with -g we emit by default DEBUG_BEGIN_STMTs (TREE_SIDE_EFFECTS
clear) and so end up with a STATEMENT_LIST containing DEBUG_BEGIN_STMT
+ e.g. the IF_STMT, while with -g0 we would end up with just the IF_STMT
alone and in that case there is no STATEMENT_LIST wrapping it.

Now, the STATEMENT_LIST has TREE_SIDE_EFFECTS set to match the IF_STMT,
but if none of the 3 operands (condition and both branches) have
TREE_SIDE_EFFECTS, genericize_if_stmt will replace the IF_STMT with
COND_EXPR without TREE_SIDE_EFFECTS, but with -g only STATEMENT_LIST
wrapping it will keep TREE_SIDE_EFFECTS.  Then during gimplification,
shortcut_cond_expr checks TREE_SIDE_EFFECTS of the operands and as it
is differennt between -g and -g0, will generate different code.

The following patch attempts to fix this by clearing TREE_SIDE_EFFECTS
on STATEMENT_LISTs that initially have it set and contain only
DEBUG_BEGIN_STMT or at most one other statement that lost TREE_SIDE_EFFECTS
during the genericization.

2020-03-26  Jakub Jelinek  <jakub@redhat.com>

	PR c++/94272
	* cp-gimplify.c (cp_genericize_r): Handle STATEMENT_LIST.

	* g++.dg/debug/pr94272.C: New test.