[utils] Fix undefined behavior in Abs helper.

This fixes undefined behavior in the arithmetic negation operation by
switching to a branch-free implementation.

R=clemensh@chromium.org
TEST=unittests/MachineOperatorReducerTest.Int32DivWithConstant

Change-Id: I518f0e4343fc331607b8bbeefd2bb06285621fe6
Reviewed-on: https://chromium-review.googlesource.com/584870Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46905}

src/utils.h

@@ -185,9 +185,15 @@ T JSMin(T x, T y) {
 
 // Returns the absolute value of its argument.
 template <typename T,
-          typename = typename std::enable_if<std::is_integral<T>::value>::type>
+          typename = typename std::enable_if<std::is_signed<T>::value>::type>
 typename std::make_unsigned<T>::type Abs(T a) {
-  return a < 0 ? -a : a;
+  // This is a branch-free implementation of the absolute value function and is
+  // described in Warren's "Hacker's Delight", chapter 2. It avoids undefined
+  // behavior with the arithmetic negation operation on signed values as well.
+  typedef typename std::make_unsigned<T>::type unsignedT;
+  unsignedT x = static_cast<unsignedT>(a);
+  unsignedT y = static_cast<unsignedT>(a >> (sizeof(T) * 8 - 1));
+  return (x ^ y) - y;
 }
 
 // Floor(-0.0) == 0.0