Fix ceil_log2(0) (PR 86644)

This PR shows a pathological case in which we try SLP vectorisation on
dead code.  We record that 0 bits of the result are enough to satisfy
all users (which is true), and that led to precision being 0 in:

static unsigned int
vect_element_precision (unsigned int precision)
{
  precision = 1 << ceil_log2 (precision);
  return MAX (precision, BITS_PER_UNIT);
}

ceil_log2 (0) returned 64 rather than 0, leading to 1 << 64, which is UB.

2018-07-25  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* hwint.c (ceil_log2): Fix comment.  Return 0 for 0.

From-SVN: r262961

gcc/ChangeLog

+2018-07-25  Richard Sandiford  <richard.sandiford@arm.com>
+
+	* hwint.c (ceil_log2): Fix comment.  Return 0 for 0.
+
 2018-07-25  Martin Liska  <mliska@suse.cz>
 
         PR middle-end/86645

gcc/hwint.c

@@ -60,12 +60,12 @@ floor_log2 (unsigned HOST_WIDE_INT x)
   return t;
 }
 
-/* Given X, an unsigned number, return the largest Y such that 2**Y >= X.  */
+/* Given X, an unsigned number, return the least Y such that 2**Y >= X.  */
 
 int
 ceil_log2 (unsigned HOST_WIDE_INT x)
 {
-  return floor_log2 (x - 1) + 1;
+  return x == 0 ? 0 : floor_log2 (x - 1) + 1;
 }
 
 /* Return the logarithm of X, base 2, considering X unsigned,