ARM: Improve SeqStringSetChar implementation.

src/arm/lithium-arm.cc

Index: src/arm/lithium-arm.cc
diff --git a/src/arm/lithium-arm.cc b/src/arm/lithium-arm.cc
index 132e1a60e83b3d46c325ea33cfe01bc656e299ac..10326df8d6aa5131fe15de028239908b985770da 100644
--- a/src/arm/lithium-arm.cc
+++ b/src/arm/lithium-arm.cc
@@ -1869,11 +1869,9 @@ LInstruction* LChunkBuilder::DoDateField(HDateField* instr) {
 
 LInstruction* LChunkBuilder::DoSeqStringSetChar(HSeqStringSetChar* instr) {
   LOperand* string = UseRegister(instr->string());
-  LOperand* index = UseRegister(instr->index());
-  LOperand* value = UseTempRegister(instr->value());
-  LSeqStringSetChar* result =
-      new(zone()) LSeqStringSetChar(instr->encoding(), string, index, value);
-  return DefineAsRegister(result);
+  LOperand* index = UseRegisterOrConstant(instr->index());
+  LOperand* value = UseRegister(instr->value());
+  return new(zone()) LSeqStringSetChar(instr->encoding(), string, index, value);
 }