middle-end/100786 - constant folding from incompatible alias

The following avoids us ICEing doing constant folding from variables
with aliases of different types.  The issue appears both in
folding and CCP and FRE can do more fancy stuff to still constant
fold cases where the load is smaller than the initializer so
defer it to there.

2022-01-20  Richard Biener  <rguenther@suse.de>

	PR middle-end/100786
	* gimple-fold.cc (get_symbol_constant_value): Only return
	values of compatible type to the symbol.

	* gcc.dg/torture/pr100786.c: New testcase.

gcc/gimple-fold.cc

@@ -291,7 +291,9 @@ get_symbol_constant_value (tree sym)
       if (val)
 	{
 	  val = canonicalize_constructor_val (unshare_expr (val), sym);
-	  if (val && is_gimple_min_invariant (val))
+	  if (val
+	      && is_gimple_min_invariant (val)
+	      && useless_type_conversion_p (TREE_TYPE (sym), TREE_TYPE (val)))
 	    return val;
 	  else
 	    return NULL_TREE;

gcc/testsuite/gcc.dg/torture/pr100786.c

@@ -0,0 +1,9 @@
+/* { dg-do compile } */
+
+const double a = 0;
+extern int b __attribute__((alias("a")));
+void inc() { b++; }
+
+const int a2 = 0;
+extern double b2 __attribute__((alias("a2")));
+void inc2() { b2+=1; }