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

runtime/vm/intrinsifier_mips.cc

Index: runtime/vm/intrinsifier_mips.cc
diff --git a/runtime/vm/intrinsifier_mips.cc b/runtime/vm/intrinsifier_mips.cc
index 7349e858169999a3a805e230028dc476c5cf8871..d148db8df12a1c6fef66d48748d0cb7b96a6c1ed 100644
--- a/runtime/vm/intrinsifier_mips.cc
+++ b/runtime/vm/intrinsifier_mips.cc
@@ -20,12 +20,12 @@ namespace dart {
 DECLARE_FLAG(bool, interpret_irregexp);
 
 // When entering intrinsics code:
-// S5: IC Data
 // S4: Arguments descriptor
 // RA: Return address
-// The S5, S4 registers can be destroyed only if there is no slow-path, i.e.
+// The S4 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_mips.h) must be preserved.
 
 #define __ assembler->