Bug 1278623 - fix emulation of POPCNT on x86. r=bbouvier

--HG--
extra : amend_source : 2bfa0d08832840203201804debbeb3dc90f9f8bc

js/src/jit/x86-shared/MacroAssembler-x86-shared-inl.h

@@ -117,7 +117,9 @@ MacroAssembler::popcnt32(Register input, Register output, Register tmp)
 
     // Equivalent to mozilla::CountPopulation32()
 
-    movl(input, output);
+    movl(input, tmp);
+    if (input != output)
+        movl(input, output);
     shrl(Imm32(1), output);
     andl(Imm32(0x55555555), output);
     subl(output, tmp);