[turbofan] Fix test of %_MathClz32 intrinsic.

This test will fail once we optimize top-level code, because the
aforementioned intrinsic doesn't perform a NumberToUint32 conversion.

R=titzer@chromium.org
TEST=mjsunit/asm/math-clz32

Review URL: https://codereview.chromium.org/1041173002

Cr-Commit-Position: refs/heads/master@{#27524}

test/mjsunit/asm/math-clz32.js

@@ -27,5 +27,5 @@ for (var i = 0; i < 32; ++i) {
 }
 for (var i = -2147483648; i < 2147483648; i += 3999773) {
   assertEquals(%MathClz32(i), f(i));
-  assertEquals(%_MathClz32(i), f(i));
+  assertEquals(%MathClz32(i), %_MathClz32(i >>> 0));
 }