middle-end/95118 - fix printing of denormal zero

This fixes printing a REAL_CST generated from value-numbering
punning some bits to a real which turns out as zero with big
negative exponent.  This causes the loop in real_to_decimal_for_mode to
never terminate.

2020-05-14  Richard Biener  <rguenther@suse.de>

	PR middle-end/95118
	* real.c (real_to_decimal_for_mode): Make sure we handle
	a zero with nonzero exponent.

	* gcc.dg/pr95118.c: New testcase.

gcc/real.c

@@ -1714,8 +1714,8 @@ real_to_decimal_for_mode (char *str, const REAL_VALUE_TYPE *r_orig,
 
 	  do_multiply (&u, &v, ten);
 
-	  /* Stop if we're now >= 1.  */
-	  if (REAL_EXP (&u) > 0)
+	  /* Stop if we're now >= 1 or zero.  */
+	  if (REAL_EXP (&u) > 0 || u.cl == rvc_zero)
 	    break;
 
 	  v = u;

gcc/testsuite/gcc.dg/pr95118.c

+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-fre" } */
+
+void a();
+void b() {
+    union {
+	int c[4];
+	long double d;
+    } e = {{0, 0, 4}};
+    a(e.d);
+}