Re-assign registers on ARM so PP and CODE_REG are below R7 (FP on iOS).

runtime/vm/intrinsifier_arm64.cc

Index: runtime/vm/intrinsifier_arm64.cc
diff --git a/runtime/vm/intrinsifier_arm64.cc b/runtime/vm/intrinsifier_arm64.cc
index 4441fe21e7cbb99e052052e4958f2ffaf46d8395..1c2620dacc27307f91320818122cc7d9aa00beed 100644
--- a/runtime/vm/intrinsifier_arm64.cc
+++ b/runtime/vm/intrinsifier_arm64.cc
@@ -20,12 +20,12 @@ namespace dart {
 DECLARE_FLAG(bool, interpret_irregexp);
 
 // When entering intrinsics code:
-// R5: IC Data
 // R4: Arguments descriptor
 // LR: Return address
-// The R5, R4 registers can be destroyed only if there is no slow-path, i.e.
+// The R4 register can be destroyed only if there is no slow-path, i.e.
 // if the intrinsified method always executes a return.
 // The FP register should not be modified, because it is used by the profiler.
+// The PP and THR registers (see constants_arm64.h) must be preserved.
 
 #define __ assembler->