MIPS64: Refine 'Remove unused byte from Map::instance_sizes field.'

Port 2027335f

Original commit message:
Note that there are currently no objects that require a pre-allocated
properties backing store, all such slots are in-object properties from
the begining. Hence {unused + pre_allocated - inobject == 0} holds.

BUG=

Review URL: https://codereview.chromium.org/1226363003

Cr-Commit-Position: refs/heads/master@{#29610}

src/mips64/builtins-mips64.cc

@@ -483,12 +483,8 @@ static void Generate_JSConstructStubHelper(MacroAssembler* masm,
                   Operand(static_cast<int64_t>(Map::kSlackTrackingCounterEnd)));
 
         // Allocate object with a slack.
-        __ lwu(a0, FieldMemOperand(a2, Map::kInstanceSizesOffset));
-        __ Ext(a0, a0, Map::kInObjectPropertiesByte * kBitsPerByte,
-               kBitsPerByte);
-        __ lwu(a2, FieldMemOperand(a2, Map::kInstanceAttributesOffset));
-        __ Ext(a2, a2, Map::kUnusedPropertyFieldsByte * kBitsPerByte,
-               kBitsPerByte);
+        __ lbu(a0, FieldMemOperand(a2, Map::kInObjectPropertiesOffset));
+        __ lbu(a2, FieldMemOperand(a2, Map::kUnusedPropertyFieldsOffset));
         __ dsubu(a0, a0, a2);
         __ dsll(at, a0, kPointerSizeLog2);
         __ daddu(a0, t1, at);