tree-optimization/109434 - bogus DSE of throwing call LHS

The byte tracking of call LHS didn't properly handle possibly
throwing calls correctly which cases bogus DSE and in turn, for the
testcase a bogus uninit diagnostic and (unreliable) wrong-code.

	PR tree-optimization/109434
	* tree-ssa-dse.cc (initialize_ao_ref_for_dse): Properly
	handle possibly throwing calls when processing the LHS
	and may-defs are not OK.

	* g++.dg/opt/pr109434.C: New testcase.

gcc/testsuite/g++.dg/opt/pr109434.C

@@ -0,0 +1,28 @@
+// { dg-do compile }
+// { dg-require-effective-target c++17 }
+// { dg-options "-O2 -Wall" }
+
+#include <optional>
+#include <stdexcept>
+
+std::optional<int> foo()
+{
+  volatile int x = 1;
+  if (x)
+    throw std::runtime_error("haha");
+  return 42;
+}
+
+int main()
+{
+  std::optional<int> optInt;
+  try {
+      // We falsely DSEd the LHS of the call even though foo throws
+      // which results in an uninitialized diagnostic
+      optInt = foo();
+  } catch (...) {
+      return optInt.has_value();
+  }
+  std::optional<double> optDbl{optInt};
+  return optDbl ? optDbl.value () : 2.0;
+}

gcc/tree-ssa-dse.cc

@@ -179,7 +179,8 @@ initialize_ao_ref_for_dse (gimple *stmt, ao_ref *write, bool may_def_ok = false)
     }
   if (tree lhs = gimple_get_lhs (stmt))
     {
-      if (TREE_CODE (lhs) != SSA_NAME)
+      if (TREE_CODE (lhs) != SSA_NAME
+	  && (may_def_ok || !stmt_could_throw_p (cfun, stmt)))
 	{
 	  ao_ref_init (write, lhs);
 	  return true;