A64: Port LSeqStringSetChar optimizations from r16707 and r17521.

src/a64/full-codegen-a64.cc

Index: src/a64/full-codegen-a64.cc
diff --git a/src/a64/full-codegen-a64.cc b/src/a64/full-codegen-a64.cc
index dd745848b088df669d8ae9af121731d5cbdbf606..2dca236faf482a986d45f7027bc8506c45e8b710 100644
--- a/src/a64/full-codegen-a64.cc
+++ b/src/a64/full-codegen-a64.cc
@@ -3255,9 +3255,10 @@ void FullCodeGenerator::EmitOneByteSeqStringSetChar(CallRuntime* expr) {
     __ Throw(kNonSmiValue);
     __ Throw(kNonSmiIndex);
     __ Bind(&both_smis);
-
+    __ SmiUntag(index);
     static const uint32_t one_byte_seq_type = kSeqStringTag | kOneByteStringTag;
-    __ EmitSeqStringSetCharCheck(string, index, one_byte_seq_type);
+    __ EmitSeqStringSetCharCheck(string, index, scratch, one_byte_seq_type);
+    __ SmiTag(index);
   }
 
   __ Add(scratch, string, SeqOneByteString::kHeaderSize - kHeapObjectTag);
@@ -3288,9 +3289,10 @@ void FullCodeGenerator::EmitTwoByteSeqStringSetChar(CallRuntime* expr) {
     __ Throw(kNonSmiValue);
     __ Throw(kNonSmiIndex);
     __ Bind(&both_smis);
-
+    __ SmiUntag(index);
     static const uint32_t two_byte_seq_type = kSeqStringTag | kTwoByteStringTag;
-    __ EmitSeqStringSetCharCheck(string, index, two_byte_seq_type);
+    __ EmitSeqStringSetCharCheck(string, index, scratch, two_byte_seq_type);
+    __ SmiTag(index);
   }
 
   __ Add(scratch, string, SeqTwoByteString::kHeaderSize - kHeapObjectTag);