MIPS64: Fix string stubs.

An innocent-looking change in 'c63e50ed Reland Update V8 DEPS.' exposed
latent bugs in SubStringStub and StringCharFromCodeGenerator.

TEST=mjsunit/string-replace, msjunit/string-split, others....
BUG=

Review URL: https://codereview.chromium.org/1233903003

Cr-Commit-Position: refs/heads/master@{#29731}

src/mips64/code-stubs-mips64.cc

@@ -3200,25 +3200,17 @@ void StringCharCodeAtGenerator::GenerateSlow(
 
 void StringCharFromCodeGenerator::GenerateFast(MacroAssembler* masm) {
   // Fast case of Heap::LookupSingleCharacterStringFromCode.
-
-  DCHECK(!a4.is(result_));
-  DCHECK(!a4.is(code_));
-
-  STATIC_ASSERT(kSmiTag == 0);
-  DCHECK(base::bits::IsPowerOfTwo32(String::kMaxOneByteCharCodeU + 1));
-  __ And(a4, code_, Operand(kSmiTagMask |
-                            ((~String::kMaxOneByteCharCodeU) << kSmiTagSize)));
-  __ Branch(&slow_case_, ne, a4, Operand(zero_reg));
-
+  __ JumpIfNotSmi(code_, &slow_case_);
+  __ Branch(&slow_case_, hi, code_,
+            Operand(Smi::FromInt(String::kMaxOneByteCharCode)));
 
   __ LoadRoot(result_, Heap::kSingleCharacterStringCacheRootIndex);
   // At this point code register contains smi tagged one_byte char code.
-  STATIC_ASSERT(kSmiTag == 0);
-  __ SmiScale(a4, code_, kPointerSizeLog2);
-  __ Daddu(result_, result_, a4);
+  __ SmiScale(at, code_, kPointerSizeLog2);
+  __ Daddu(result_, result_, at);
   __ ld(result_, FieldMemOperand(result_, FixedArray::kHeaderSize));
-  __ LoadRoot(a4, Heap::kUndefinedValueRootIndex);
-  __ Branch(&slow_case_, eq, result_, Operand(a4));
+  __ LoadRoot(at, Heap::kUndefinedValueRootIndex);
+  __ Branch(&slow_case_, eq, result_, Operand(at));
   __ bind(&exit_);
 }
 
@@ -3306,11 +3298,8 @@ void SubStringStub::Generate(MacroAssembler* masm) {
 
   __ ld(a2, MemOperand(sp, kToOffset));
   __ ld(a3, MemOperand(sp, kFromOffset));
-// Does not needed?
-//  STATIC_ASSERT(kFromOffset == kToOffset + 4);
+
   STATIC_ASSERT(kSmiTag == 0);
-// Does not needed?
-// STATIC_ASSERT(kSmiTagSize + kSmiShiftSize == 1);
 
   // Utilize delay slots. SmiUntag doesn't emit a jump, everything else is
   // safe in this case.
@@ -3503,6 +3492,7 @@ void SubStringStub::Generate(MacroAssembler* masm) {
   // a1: instance type
   // a2: length
   // a3: from index (untagged)
+  __ SmiTag(a3);
   StringCharAtGenerator generator(v0, a3, a2, v0, &runtime, &runtime, &runtime,
                                   STRING_INDEX_IS_NUMBER, RECEIVER_IS_STRING);
   generator.GenerateFast(masm);