Resync inline implementation of ceil_log2 (PR 86506)

In r262961 I only updated the out-of-line copy of ceil_log2.  This patch
applies the same change to the other (inline) one.

2018-07-30  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR tree-optimization/86506
	* hwint.h (ceil_log2): Resync with hwint.c implementation.

From-SVN: r263064

gcc/ChangeLog

+2018-07-30  Richard Sandiford  <richard.sandiford@arm.com>
+
+	PR tree-optimization/86506
+	* hwint.h (ceil_log2): Resync with hwint.c implementation.
+
 2018-07-30  Ilya Leoshkevich  <iii@linux.ibm.com>
 
         PR target/86547

gcc/hwint.h

@@ -242,7 +242,7 @@ floor_log2 (unsigned HOST_WIDE_INT x)
 static inline int
 ceil_log2 (unsigned HOST_WIDE_INT x)
 {
-  return floor_log2 (x - 1) + 1;
+  return x == 0 ? 0 : floor_log2 (x - 1) + 1;
 }
 
 static inline int