Cleanup of class id loading sequences.

runtime/vm/stub_code_arm64.cc

Index: runtime/vm/stub_code_arm64.cc
===================================================================
--- runtime/vm/stub_code_arm64.cc	(revision 38283)
+++ runtime/vm/stub_code_arm64.cc	(working copy)
@@ -1374,8 +1374,8 @@
   __ sub(R7, R7, Operand(1));
 
   // R0 <- [SP + (R7 << 3)]
-  __ ldr(R0, Address(SP, R7, UXTX, Address::Scaled));
-  __ LoadTaggedClassIdMayBeSmi(R0, R0);
+  __ ldr(R1, Address(SP, R7, UXTX, Address::Scaled));
+  __ LoadTaggedClassIdMayBeSmi(R0, R1);
 
   // R7: argument_count - 1 (untagged).
   // R0: receiver's class ID (smi).
@@ -1388,8 +1388,8 @@
       // If not the first, load the next argument's class ID.
       __ AddImmediate(R0, R7, -i, kNoPP);
       // R0 <- [SP + (R0 << 3)]
-      __ ldr(R0, Address(SP, R0, UXTX, Address::Scaled));
-      __ LoadTaggedClassIdMayBeSmi(R0, R0);
+      __ ldr(R1, Address(SP, R0, UXTX, Address::Scaled));
+      __ LoadTaggedClassIdMayBeSmi(R0, R1);
       // R0: next argument class ID (smi).
       __ LoadFromOffset(R1, R6, i * kWordSize, kNoPP);
       // R1: next class ID to check (smi).
@@ -1405,8 +1405,8 @@
   __ Bind(&update);
   // Reload receiver class ID.  It has not been destroyed when num_args == 1.
   if (num_args > 1) {
-    __ ldr(R0, Address(SP, R7, UXTX, Address::Scaled));
-    __ LoadTaggedClassIdMayBeSmi(R0, R0);
+    __ ldr(R1, Address(SP, R7, UXTX, Address::Scaled));
+    __ LoadTaggedClassIdMayBeSmi(R0, R1);
   }
 
   const intptr_t entry_size = ICData::TestEntryLengthFor(num_args) * kWordSize;