Bug 1500231: Fix register allocation of copysign on AVX; r=sunfish

--HG--
extra : rebase_source : 99a85227520130cc006fbb4ff8b6bb3d7a1a03f6
extra : amend_source : 81b0b96934c487246b0188d63ce4b3c02a82b417

js/src/jit/x86-shared/Lowering-x86-shared.cpp

@@ -704,10 +704,11 @@ LIRGenerator::visitCopySign(MCopySign* ins)
 
     // As lowerForFPU, but we want rhs to be in a FP register too.
     lir->setOperand(0, useRegisterAtStart(lhs));
-    lir->setOperand(1, lhs != rhs ? useRegister(rhs) : useRegisterAtStart(rhs));
     if (!Assembler::HasAVX()) {
+        lir->setOperand(1, lhs != rhs ? useRegister(rhs) : useRegisterAtStart(rhs));
         defineReuseInput(lir, ins, 0);
     } else {
+        lir->setOperand(1, useRegisterAtStart(rhs));
         define(lir, ins);
     }
 }