Bug 925088 - SpiderMonkey: Micro-optimize x64's testStringTruthy. r=mjrosen

js/src/jit/x64/MacroAssembler-x64.h

@@ -1103,14 +1103,11 @@ class MacroAssemblerX64 : public MacroAssemblerX86Shared
         testl(operand.valueReg(), operand.valueReg());
         j(truthy ? NonZero : Zero, label);
     }
-    // This returns the tag in ScratchReg.
     Condition testStringTruthy(bool truthy, const ValueOperand &value) {
         unboxString(value, ScratchReg);
 
         Operand lengthAndFlags(ScratchReg, JSString::offsetOfLengthAndFlags());
-        movq(lengthAndFlags, ScratchReg);
-        shrq(Imm32(JSString::LENGTH_SHIFT), ScratchReg);
-        testq(ScratchReg, ScratchReg);
+        testq(lengthAndFlags, Imm32(-1 << JSString::LENGTH_SHIFT));
         return truthy ? Assembler::NonZero : Assembler::Zero;
     }