PPC: fix input reg clobbering on atomic64

R=joransiu@ca.ibm.com

Change-Id: Ic9a6a9055e3f05ef64723d995228673ee48eb994
Reviewed-on: https://chromium-review.googlesource.com/c/1296682Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#56968}

src/compiler/ppc/code-generator-ppc.cc

@@ -652,16 +652,16 @@ void EmitWordLoadPoisoningIfNeeded(CodeGenerator* codegen, Instruction* instr,
     __ bne(&binop, cr0);                                                     \
   } while (false)
 
-#define ASSEMBLE_ATOMIC_BINOP_SIGN_EXT(bin_inst, load_inst,                    \
-                                       store_inst, ext_instr)                  \
+#define ASSEMBLE_ATOMIC_BINOP_SIGN_EXT(bin_inst, load_inst, store_inst,      \
+                                       ext_instr)                            \
   do {                                                                       \
     MemOperand operand = MemOperand(i.InputRegister(0), i.InputRegister(1)); \
     Label binop;                                                             \
     __ bind(&binop);                                                         \
     __ load_inst(i.OutputRegister(), operand);                               \
     __ ext_instr(i.OutputRegister(), i.OutputRegister());                    \
-    __ bin_inst(i.InputRegister(2), i.OutputRegister(), i.InputRegister(2));   \
-    __ store_inst(i.InputRegister(2), operand);                                \
+    __ bin_inst(kScratchReg, i.OutputRegister(), i.InputRegister(2));        \
+    __ store_inst(kScratchReg, operand);                                     \
     __ bne(&binop, cr0);                                                     \
   } while (false)