c: Fix ICE with cast to VLA [93576]

The following testcase ICEs, because the PR84305 changes try to evaluate
the size earlier.  If size has side-effects, that is desirable, and the
side-effects will actually be wrapped in a SAVE_EXPR.  The problem on this
testcase is that there are no side-effects, and c_fully_fold doesn't fold
those COMPOUND_EXPRs to constant, and while before gimplification we unshare
trees found in the expressions, the unsharing doesn't involve TYPE_SIZE etc.
of used types.  Gimplification is destructive though, so when we gimplify
the two nested COMPOUND_EXPRs and then try to gimplify it the second time
for the TYPE_SIZEs, we ICE.
Now, we could use unshare_expr in what we push to *expr, SAVE_EXPRs and
their operands in there aren't unshared, but I really don't see a point of
evaluating expressions that don't have side-effects before, so instead
this just pushes there expressions that do have side-effects.

2020-02-13  Jakub Jelinek  <jakub@redhat.com>

	PR c/93576
	* c-decl.c (grokdeclarator): If this_size_varies, only push size into
	*expr if it has side effects.

	* gcc.dg/pr93576.c: New test.

gcc/c/ChangeLog

@@ -1,3 +1,9 @@
+2020-02-13  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c/93576
+	* c-decl.c (grokdeclarator): If this_size_varies, only push size into
+	*expr if it has side effects.
+
 2020-01-30  Jeff Law  <law@redhat.com>
 
 	PR c/88660

gcc/c/c-decl.c

@@ -6512,11 +6512,14 @@ grokdeclarator (const struct c_declarator *declarator,
 		  }
 		if (this_size_varies)
 		  {
-		    if (*expr)
-		      *expr = build2 (COMPOUND_EXPR, TREE_TYPE (size),
-				      *expr, size);
-		    else
-		      *expr = size;
+		    if (TREE_SIDE_EFFECTS (size))
+		      {
+			if (*expr)
+			  *expr = build2 (COMPOUND_EXPR, TREE_TYPE (size),
+					  *expr, size);
+			else
+			  *expr = size;
+		      }
 		    *expr_const_operands &= size_maybe_const;
 		  }
 	      }

gcc/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2020-02-13  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c/93576
+	* gcc.dg/pr93576.c: New test.
+
 2020-02-13  Sandra Loosemore  <sandra@codesourcery.com>
 
 	* g++.dg/cpp0x/constexpr-static13.C:

gcc/testsuite/gcc.dg/pr93576.c

@@ -0,0 +1,10 @@
+/* PR c/93576 */
+/* { dg-do compile } */
+/* { dg-options "" } */
+
+void
+foo (void)
+{
+  int b[] = { 0 };
+  (char (*)[(1, 7, 2)]) 0;
+}