ppc64: fix clobbering issue on input register

R=joransiu@ca.ibm.com

Change-Id: Id53be07f0f980dc03b1106c09b6fddbf94d31088
Reviewed-on: https://chromium-review.googlesource.com/c/1262937Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#56392}

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

@@ -670,8 +670,8 @@ void EmitWordLoadPoisoningIfNeeded(CodeGenerator* codegen, Instruction* instr,
     Label binop;                                                             \
     __ bind(&binop);                                                         \
     __ load_inst(i.OutputRegister(), operand);                               \
-    __ 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)